Port to KDE Frameworks 5.

Squashed commit of the kde-frameworks-5 branch, which consisted of
commit 5f8db001 (message repeated below) and some other minor ones.

Affects apps/marble-kde, bindings (not tested), doc, plasmarunner,
plasmoid (deactivated as ktimezonewidget is now gone), thumbcreator.

Also cleans up cmake files: The QTONLY options is gone completely.
KDE parts are only included where necessary, most notably not in
src/lib. Since users might still pass it, QTONLY=TRUE is forwarded
to WITH_KF5=FALSE which matches the old behavior most closely. The new
behavior is better by default however, not erroring out if KF5 is
missing, but instead building Qt parts only.
Additionally the QT5BUILD option is sanitized to behave as originally
planned: You don't have to pass it anymore when both Qt4 and Qt5 are
installed, but cmake figures out which of the two to use and the other
one is disabled (does not interfere anymore).

Compiles without warnings (aside automoc) and without kde4support
libraries. I'm pretty sure however that I broke some parts at runtime
for now, e.g. config saving and installation/loading of some parts.
I18n is not tested as well. Needs to be taken care of in subsequent
commits.