Add a configurable option to hide the tab bar in Viewer when only one tab is open.

BUG: 326651
FIXED: [ 326651 ] Hide tab bar in internal editor/viewer, when shown only one tab

Add to Konfigurator two options to set foreground and background colors to be
applied to the rename field.

BUG: 301034
BUG: 414968
FIXED: [ 301034 ] Rename input field doesn't respect text[NORMAL] when GTK theme is used
FIXED: [ 414968 ] Using provided custom colour package Midnight Commander, renaming files is difficult, as line colours are very pale.
ADDED: In Konfigurator, two options are added to set foreground and background colors to the rename field.
GUI: Two new options ("Rename background" and "Rename foreground") are seen by users.

Fixed all the warnings generated by Qt-5.14.2 + GCC-9.3.0 setup
to restore the clean build.

Examples of the warnings:
* ‘QList QSet::toList() const [with T = QUrl]’ is deprecated: Use values() instead. [-Wdeprecated-declarations]
* ‘QSet QList::toSet() const [with T = QUrl]’ is deprecated: Use QSet(list.begin(), list.end()) instead. [-Wdeprecated-declarations]
* ‘static QSet QSet::fromList(const QList&) [with T = QString]’ is deprecated: Use QSet(list.begin(), list.end()) instead. [-Wdeprecated-declarations]
* implicitly-declared ‘FilterSettings::FilterSettings(const FilterSettings&)’ is deprecated [-Wdeprecated-copy]
* ‘int QTime::elapsed() const’ is deprecated: Use QElapsedTimer instead [-Wdeprecated-declarations]
* ‘void QTime::start()’ is deprecated: Use QElapsedTimer instead [-Wdeprecated-declarations]
* ‘void QTextEdit::setTabStopWidth(int)’ is deprecated [-Wdeprecated-declarations]

Discussion: !7

Remove the .arcconfig file, as in websites/krusader-org!2

Adds an option to close a Viewer tab using the middle mouse click.

FIXED: [ 228461 ] KrViewer - Middle mouse button to close tabs in default settings
BUG: 228461

Allows the user to run .desktop instead of opening it in the external editor.

As a .desktop file could lead to security issues (evil Exec=), KIO will warn
the user when he tries to launch it from an unknown location (see
XDG_DATA_DIRS), setting the file with executable bit if the user confirms.

FIXED: [ 394938 ] Can't run .desktop files
BUG: 394938

Update the address of the repository browser in the README file, because
until today https://cgit.kde.org/krusader.git had not been updated
(since the beginning of the migration to Gitlab), and the use
of invent.kde.org is preferred nowadays.

In version 5.69 of KIO,

KIO::stat(const QUrl&, KIO::StatJob::StatSide, short int, JobFlags)

is deprecated in favor of

KIO::statDetails(const QUrl, KIO::StatJob::StatSide, KIO::StatDetails, JobFlags)

It can use the original KIO URLs. No need for using temporary local files.
And remove the obsolete space character check for file paths for KDiff3.

CCBUG: 401064

Differential Revision: https://phabricator.kde.org/D24987

Timur Guzairov ("tguzairov") wrote the main part of the present commit:
Parts::ReadOnlyPart::completed signal is connected after openUrl has been
called, and it won't be emited if openUrl was fast enough. In this case
openUrlFinished won't be emited, which could, for example, cause the text not
receiving a focus.

Differential Revision: https://phabricator.kde.org/D26280

Differential Revision: https://phabricator.kde.org/D29611

Differential Revision: https://phabricator.kde.org/D29609

Konfigurator: Avoid needing to apply `setWhatsThis()` and `setToolTip()` before calling `createURLRequester()`

Modify `createURLRequester(...)` in order to require a QLabel as an argument,
therefore applying `setWhatsThis(toolTip)` and `setToolTip()` to
that QLabel is done automatically (and those steps do not have to
be remembered by developers).

Differential Revision: https://phabricator.kde.org/D29608

Konfigurator: Avoid needing to apply `setWhatsThis()` and `setToolTip()` before calling `createFontChooser()`

Modify `createFontChooser(...)` in order to require a QLabel as an argument,
therefore applying `setWhatsThis(toolTip)` and `setToolTip()` to
that QLabel is done automatically (and those steps do not have to
be remembered by developers).

Differential Revision: https://phabricator.kde.org/D29607

Konfigurator: Avoid needing to apply `setWhatsThis()` and `setToolTip()` before calling `createEditBox()`

Modify `createEditBox(...)` in order to require a QLabel as an argument,
therefore applying `setWhatsThis(toolTip)` and `setToolTip()` to
that QLabel is done automatically (and those steps do not have to
be remembered by developers).

Differential Revision: https://phabricator.kde.org/D29606

Konfigurator: Avoid needing to apply `setWhatsThis()` and `setToolTip()` before calling `createComboBox()`

Modify `createComboBox(...)` in order to require a QLabel as an argument,
therefore applying `setWhatsThis(toolTip)` and `setToolTip()` to
that QLabel is done automatically (and those steps do not have to
be remembered by developers).

Differential Revision: https://phabricator.kde.org/D29605

Konfigurator: Avoid needing to apply `setWhatsThis()` and `setToolTip()` before calling `createSpinBox()`

Modify `createSpinBox(...)` in order to require a QLabel as an argument,
therefore applying `setWhatsThis(toolTip)` and `setToolTip()` to
that QLabel is done automatically (and those steps do not have to
be remembered by developers).

Differential Revision: https://phabricator.kde.org/D29546