Store clipboard before exiting native GTK dialog

Gtk library usually takes care of this when the main
Gtk loop ends, but since Gtk's main even loop is not
used in QGtk2Dialog we have to store clipboard's content
with help of gtk_clipboard_store(). This function
sends a SAVE_TARGETS request to X11 clipboard manager
to save clipboards contents as required by ICCCM.

Task-number: QTBUG-34475
Change-Id: If784c425ea4a36ec1c3a8ddc0cdec080f57681a5
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>