The separate filepicker executable 'lo_kde5filepicker'
used by the gtk3_kde5 VCL plugin needs the KF5 libraries
(in particular the 'KFileWidget' class) in order to
provide custom controls in the Plasma-native QFileDialog.

The gtk3 side ('lo_kde5filepicker') itself doesn't use
any functionality from KF5 libraries itself and therefore
didn't link against those.

When determining which VCL plugin to use on LibreOffice
start, a 'dlopen()' is used on the libraries of the
potential VCL plugins, which fails if linked libraries
are not available, so unsuitable VCL plugin are skipped.

This mechanism didn't work for the gtk3_kde5 VCL plugin
if KF5 libraries were not installed, since the 'dlopen()'
on 'libvclplug_gtk3_kde5lo.so' would succeed, but starting
the 'lo_kde5filepicker' process would later fail at runtime
due to the missing libraries.

To avoid this, make 'libvclplug_gtk3_kde5lo.so' link
against KF5 libraries as well to prevent gtk3_kde5 from
being used unless all required libraries are actually present.

Add a dummy KFileWidget use in Gtk3KDE5FilePicker to prevent
smart linkers from not linking unused libraries despite linker
flags.

While this is a hack, it seems to be acceptable, in
particular since gtk3_kde5 is more meant as a temporary
solution supposed to be dropped at a later point in time
in favor of the pure kde5 VCL plugin.

Change-Id: I3a37daaeb1307b4f24f6fd75d7b6ec5ffaade0fd
Reviewed-on: https://gerrit.libreoffice.org/71237Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>
Tested-by: Jenkins