And use RunInMain for both SalInstances and for both picker types,
as there is no reason to assume just the file and not the folder
picker can be called from the non-GUI thread.

Little drawback is the inclusion of Qt5FilePicker header in the
Qt5Instance header, as Qt's enums aren't forward-declarable.

Change-Id: Ie170d247a76134df9aff835393c71c9d6e907d32
Reviewed-on: https://gerrit.libreoffice.org/73416Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>