This is mostly a copy of the KDE4 backend ported to Qt5/KF5.

One difference is that this code will initialize the QApplication
on-demand, when it's not yet available. This will allow us to use
this desktop backend also within a vclplug that does not use Qt
itself, such as the upcoming Gtk3/KDE5 hybrid.

Change-Id: I5cf96ac5729608c82a58eead6723a38f014ba875
Reviewed-on: https://gerrit.libreoffice.org/47716Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>