Cleanup GTK_CHECK_VERSION and unify code where possible

* Use GtkAllocation for both Gtk+ versions, added in 2.18
* Bump gdk-pixbuf required to 2.22 (release 2010)
* GDK_DISPLAY_XDISPLAY | Since always
* gdk_display_get_default | Since 2.22
* gdk_window_get_screen | Since 2.24
* gtk_widget_get_window | Since 2.14
Use g_object_unref in callback for both Gtk+ versions