Startup: Do not crash when reporting check problems.

* glom/main.c: Instantiate the Application earlier, to ensure
  that GTK+ is ready for use.
  Also call gtk_init() explicitly because Gtk::Application needs
  a fix for this anyway, and it would be nice to use this fix
  in Glom versions that do not need a newer gtkmm.

  Bug #680411 (Bjørn Lie, Dominique Leuenberger)