Add testWM: a minimal tiling WM built on wlroots++

Master/stack tiling with Alt as modifier: Alt+Enter spawns a terminal
(kitty), Alt+Q closes, Alt+J/Alt+K cycle focus, click-to-focus. Reads
~/.config/testwm/testwm.conf for terminal/modifier/layout. Verified
headless: two clients receive 640x720 master + 640x720 stack configures.

Wrapper additions: Cursor motion/button/axis/frame signal hooks,
Seat::keyboard_notify_enter, Toplevel::close().

include/wlr/seat.hpp

@@ -27,8 +27,17 @@ public:
 	}
 	void warp(double x, double y) { wlr_cursor_warp(m_cursor, nullptr, x, y); }
 
+	void on_motion(std::function<void(void*)> cb) { m_motion.connect(&m_cursor->events.motion, std::move(cb)); }
+	void on_button(std::function<void(void*)> cb) { m_button.connect(&m_cursor->events.button, std::move(cb)); }
+	void on_axis(std::function<void(void*)> cb) { m_axis.connect(&m_cursor->events.axis, std::move(cb)); }
+	void on_frame(std::function<void(void*)> cb) { m_frame.connect(&m_cursor->events.frame, std::move(cb)); }
+
 private:
 	struct wlr_cursor* m_cursor;
+	SignalSink m_motion;
+	SignalSink m_button;
+	SignalSink m_axis;
+	SignalSink m_frame;
 };
 
 class Keyboard {
@@ -71,6 +80,11 @@ public:
 	void keyboard_notify_key(uint32_t time, uint32_t keycode, enum wl_keyboard_key_state state) {
 		wlr_seat_keyboard_notify_key(m_seat, time, keycode, state);
 	}
+	void keyboard_notify_enter(struct wlr_surface* surface,
+			uint32_t* keycodes = nullptr, size_t num_keycodes = 0,
+			struct wlr_keyboard_modifiers* modifiers = nullptr) {
+		wlr_seat_keyboard_notify_enter(m_seat, surface, keycodes, num_keycodes, modifiers);
+	}
 	void pointer_notify_motion(uint32_t time, double sx, double sy) {
 		wlr_seat_pointer_notify_motion(m_seat, time, sx, sy);
 	}

include/wlr/xdg_shell.hpp

@@ -32,6 +32,7 @@ public:
 	uint32_t set_size(int w, int h) { return wlr_xdg_toplevel_set_size(m_toplevel, w, h); }
 	uint32_t set_activated(bool activated) { return wlr_xdg_toplevel_set_activated(m_toplevel, activated); }
 	uint32_t set_maximized(bool maximized) { return wlr_xdg_toplevel_set_maximized(m_toplevel, maximized); }
+	void close() { wlr_xdg_toplevel_send_close(m_toplevel); }
 
 	void on_request_move(std::function<void(void*)> cb) {
 		m_requestMove.connect(&m_toplevel->events.request_move, std::move(cb));