Use G_SOURCE_CONTINUE/REMOVE

Now that GLib provides these macros, we should use them
to make the code more readable.

gdk/broadway/gdkdisplay-broadway.c

@@ -483,7 +483,7 @@ process_input_idle_cb (GdkBroadwayDisplay *display)
 {
   display->process_input_idle = 0;
   process_input_messages (display);
-  return FALSE;
+  return G_SOURCE_REMOVE;
 }
 
 static void

gdk/gdk.c

@@ -895,7 +895,7 @@ gdk_threads_add_idle (GSourceFunc    function,
  *    
  *    self->timeout_id = 0;
  *    
- *    return FALSE;
+ *    return G_SOURCE_REMOVE;
  * }
  *  
  * static void some_widget_do_stuff_later (SomeWidget *self)

gdk/x11/gdkapplaunchcontext-x11.c

@@ -221,7 +221,7 @@ startup_timeout (void *data)
     std->timeout_id = g_timeout_add_seconds ((min_timeout + 500)/1000, startup_timeout, std);
 
   /* always remove this one, but we may have reinstalled another one. */
-  return FALSE;
+  return G_SOURCE_REMOVE;
 }