Avoid creating gtkrc-2.0 if it does not exist

This MR would fix the following 2 bugs: Plasma does not follow GTK2_RC_FILES environment variable and would create ~/.gtkrc-2.0 every time on login.

• https://bugs.kde.org/show_bug.cgi?id=415770
• https://bugs.kde.org/show_bug.cgi?id=417534

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Here is the more detailed explanation:

gmenu-dbusmenu-proxy hardcoded gtkrcPath as QDir::homePath() + QLatin1String("/.gtkrc-2.0"):

https://invent.kde.org/plasma/plasma-workspace/-/blob/v5.27.1/gmenu-dbusmenu-proxy/menuproxy.cpp#L163

but it's not creating the file if it's not present. kde-gtk-config's Gtk2ConfigEditor::removeLegacyStrings() is the cause:

https://invent.kde.org/plasma/kde-gtk-config/-/blob/v5.27.1/kded/config_editor/gtk2.cpp#L54

it uses Utils::readFileContents(QFile &file)

https://invent.kde.org/plasma/kde-gtk-config/-/blob/v5.27.1/kded/config_editor/utils.cpp#L19

which calls file.open(QIODevice::ReadWrite | QIODevice::Text), and the mode ReadWrite means to create the file if it's not exist:

https://doc.qt.io/qt-5/qfile.html#open

So at startup, kde-gtk-config launches and a empty ~/.gtkrc-2.0 file is created, then gmenu-dbusmenu-proxy watchs the file creation

https://invent.kde.org/plasma/plasma-workspace/-/blob/v5.27.1/gmenu-dbusmenu-proxy/menuproxy.cpp#L106

and write the content to the file.

Furthermore, even if ~/.gtkrc-2.0 does not exist, Gtk2ConfigEditor::removeLegacyStrings() would still write "modified" content to the file

https://invent.kde.org/plasma/kde-gtk-config/-/blob/v5.27.1/kded/config_editor/gtk2.cpp#L76

but in that case, it would just create an empty file, for gmenu-dbusmenu-proxy to watch and write something to it.