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/seat.hpp

@@ -85,6 +85,11 @@ public:
 			struct wlr_keyboard_modifiers* modifiers = nullptr) {
 		wlr_seat_keyboard_notify_enter(m_seat, surface, keycodes, num_keycodes, modifiers);
 	}
+	void keyboard_clear_focus() { wlr_seat_keyboard_clear_focus(m_seat); }
+	void pointer_notify_enter(struct wlr_surface* surface, double sx, double sy) {
+		wlr_seat_pointer_notify_enter(m_seat, surface, sx, sy);
+	}
+	void pointer_clear_focus() { wlr_seat_pointer_clear_focus(m_seat); }
 	void pointer_notify_motion(uint32_t time, double sx, double sy) {
 		wlr_seat_pointer_notify_motion(m_seat, time, sx, sy);
 	}