Summary:
* make static methods member methods for access to members
  (e.g. default font member)
* remove default arg value QFont(QLatin1String("Arial")
* reuse same QFont object more (implicitely shared data)

Seems vector tiles are cached somehow, so a new font is
not picked up instantly

Reviewers: sanjibanb, nienhueser, #marble

Reviewed By: nienhueser, #marble

Differential Revision: https://phabricator.kde.org/D2598

Missed to push another needed patch to KPackage to make this work,
so will be only a later KF5 version where this is fixed hopefully.

This reverts commit fb427298.

Summary:
* subclass DeclarativeDataPlugin replaced that need
* allows to drop dep of libmarblewidget to Qt5::Quick again

Follow-up for c9bf0b77

Reviewers: sanjibanb, nienhueser, #marble

Reviewed By: nienhueser, #marble

Differential Revision: https://phabricator.kde.org/D2714

(cherry-picked from branch Applications/16.08, resolved conflicts)

Test Plan: Works with OSM and Moon legends + Russian marble_qt.qm.

Reviewers: nienhueser, kossebau

Reviewed By: kossebau

Subscribers: #marble

Differential Revision: https://phabricator.kde.org/D2566

pass a GeoDataPlacemark rather than a visual category, so in the future more magic can happen behind the scene

Only done for methods in installed header files

Summary:
Some code getting data from OsmPlacemarkData instances currently
does a double-lookup with data.containsTagKey(x) & data.tagValue(x).
As OsmPlacemarkData already exposes begin and end iterators for the
tags, adding a findTag(const QString &key) to allow single-lookup
of a tag on conditional access to its value  seems matching.

Resulting code trades readability (IMHO) for speed, but for a library this
option seems to make sense.

Reviewers: shentey, sanjibanb, nienhueser, #marble

Reviewed By: nienhueser, #marble

Differential Revision: https://phabricator.kde.org/D2684

CCBUG: 368219

* avoid multiple lookup (data.containsTagKey(x) & data.tagValue(x))
  when both an existing empty QString value for a tag and a non-existing
  tag are handled the same, an empty QString() is not expensive to use

Without this memory consumption raises continously and exceeds 32 GB
already when working on level 7.

Lazily create QWidget, QPixmap, QFont... dependend stuff