Use GDK_WINDOW_XID for gtk+2 and gtk+3

It was introduced in gdk2 already so clean up the code.
Use gdk_window_get_width also for gtk2

It was introduced in gtk+-2.24
Use gtk_list_store_insert_with_values also for gtk2

It was introduced in gtk+-2.6.
Use gtk_widget_set_size_request which is also available in gtk+2
Use gdk_window_add_filter also for gtk+2
Remove all calls gtk_dialog_set_has_separator
Use gdk_x11_window_lookup_for_display also in gtk+2
Drop support for gtk+ < 2.24
Use gdk_x11_set_sm_client_id also for gtk+2 (since 2.24)
Use GDK_WINDOW_XID
Drop support for glib < 2.36
Use g_spawn_async also for gtk+2
Use pid in g_spawn_async like gnome-panel does, fixes warning
Use gtk_combo_box_text_new for gtk+2

It was introduced in gtk+-2.24
Remove/comment unused variables
Bump GLIB version