add xdg decoration, scene/tree factories, geometry + seat focus

more wrapper soup. xdg decoration manager, scene/tree node factories so
callers stop touching raw wlr_scene_*_create, a box geometry helper, and
seat pointer/keyboard focus clearing. dragging myself through it.

include/wlr/wlr.hpp

@@ -20,7 +20,9 @@ extern "C" {
 #include <wlr/types/wlr_pointer.h>
 #include <wlr/types/wlr_cursor.h>
 #include <wlr/types/wlr_xdg_shell.h>
+#include <wlr/types/wlr_xdg_decoration_v1.h>
 #include <wlr/types/wlr_data_device.h>
+#include <wlr/util/box.h>
 #include <wlr/backend.h>
 #include <wlr/render/wlr_renderer.h>
 #include <wlr/render/allocator.h>