Adhere to what gtk3's file picker does as well.
Threading is involved in Gtk3KDE5FilePickerIpc::execute().

Change-Id: I4fa0a12f46ed13ef04211cb8577e77d1db5e8eae
Reviewed-on: https://gerrit.libreoffice.org/64502
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>