Bug #63962 Dynamically scan the config directory for icon themes

The hard-coded icon themes have been replaced by a dynamic list
which is filled by scanning the config directory

Conflicts:
	include/vcl/settings.hxx
	vcl/source/app/settings.cxx
	vcl/source/window/window.cxx

Change-Id: Ie3680ffe27d06e375acf22753e036cb6ddabc4ed
Reviewed-on: https://gerrit.libreoffice.org/7935
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>