Cleanup GTK_CHECK_VERSION and unify gtk+2/3 code where possible

Replace functions available in gtk+2 and gtk+3
gdk_window_get_* | since 2.24
gtk_widget_get_allocation | since 2.18
gtk_widget_get_window | since 2.14
gtk_window_get_group | since 2.10
gtk_widget_get_mapped | since 2.20
gtk_widget_get_realized | since 2.20
gdk_window_get_display | since 2.24

Remove deprecated GtkNotebookPage and check.

pluma/smclient/eggsmclient-xsmp.c

@@ -367,11 +367,7 @@ sm_client_xsmp_startup (EggSMClient *client,
       free (ret_client_id);
 
       gdk_threads_enter ();
-#if GTK_CHECK_VERSION (3, 0, 0)
       gdk_x11_set_sm_client_id (xsmp->client_id);
-#else
-      gdk_set_sm_client_id (xsmp->client_id);
-#endif
       gdk_threads_leave ();
 
       g_debug ("Got client ID \"%s\"", xsmp->client_id);