Port to GtkApplication API's
    
    Most of this patch consists in code being shuffled around to fit the
    expected flow while using the new APIs. I tried my best to make this
    patch the less intrusive as possible. Main changes are:
    
    - Updated build requirements
       * glib version 2.38
       * gtk+ version 3.10
       * gio
    
    - VirtViewerApp is now a subclass of GtkApplication.
      Some mainloop calls were replaced:
       * gtk_main() -> g_application_run()
       * gtk_quit() -> g_application_quit()
    
    - Unified command line option handling.
      The logic has moved from the main functions and split in common
      options, and specific ones for each application. With this, the main
      functions were highly simplified, and now basically responsible for
      instantiating the App object and running the main loop.
    
    - All Window objects must be associated with the Application.
      With this, there is no need to emit our own 'window-added'/'window-
      removed' signals, as those will be emited by GtkApplication whenever
      gtk_application_add_window() and gtk_application_remove_window() are
      called. Also, 'window-removed' was not being used anywhere.
    
    Signed-off-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>