Bug 677776 - filter popup windows get hidden behind main image window

On OSX, call [NSApp activateIgnoringOtherApps] when a plug-in dialog
is shown, so the plug-in process becomes the active app, and the
dialog gets focussed.

In order to avoid doing this in GimpDialog (which is also used in
the core), do it in gimp_ui_init() which all interactive plug-ins
call, and when gimp_temp_proc_run() is called interactively, to
catch repeated activation of an already running plug-in.

Also, set GimpDialog's initial position to GTK_WIN_POS_CENTER,
or they will pop up in the top left corner.

Inspired by patches from Simone Karin Lehmann and Daniel Sabo.

libgimpwidgets/gimpdialog.c

@@ -149,6 +149,10 @@ gimp_dialog_class_init (GimpDialogClass *klass)
 static void
 gimp_dialog_init (GimpDialog *dialog)
 {
+#ifdef GDK_WINDOWING_QUARTZ
+  gtk_window_set_position (GTK_WINDOW (dialog), GTK_WIN_POS_CENTER);
+#endif
+
   g_signal_connect (dialog, "response",
                     G_CALLBACK (gimp_dialog_response),
                     NULL);