Stop reading commands from the pipe on kde5 side once
the "Quit" command has been sent, in order to have
the thread that is reading commands from stdin finish
properly.

Join the thread in the 'FilePickerIpc' destructor, rather than
just deleting it while it may still be running, which
resulted in 'terminate()' being called.

Change-Id: Ia184987e7994cc1de0208ff2757a3cf06c8b7194
Reviewed-on: https://gerrit.libreoffice.org/63835
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>