This introduces a new thread that takes care of handling the IPC
command and result for executing the file picker
('Commands::Execute'), which is the only command in the file
picker that may block for a longer time until it receives its
result from the kde5 side (namely, when the dialog has been
closed).

While the file dialog is being executed, activate a dummy GTK dialog
that also takes care of handling events in its main loop as long
as the file dialog is shown. The dummy dialog is closed together
with the KDE file dialog.
(Since the actual KDE file dialog is run as a separate process,
this one is mostly "transparent" to the soffice process from the
point of view of a dialog.)

This allows dropping the custom event processing previously done
in 'Gtk3KDE5FilePickerIpc::readResponse()' that had the potential
to cause all kinds of problems, e.g. when another event related to
the file picker was triggered from a Java process via UNO.

Change-Id: I3d663253f09320f7a8e0d9ec32a8fd6ec191c189
Reviewed-on: https://gerrit.libreoffice.org/61253
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>