launcher: Collect launcher state in new struct weston_launcher

We're going to add a bit more launcher state, so start out by creating
a new struct weston_launcher we can track it in.
diff --git a/src/compositor.h b/src/compositor.h
index a523c83..0feca41 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -590,7 +590,7 @@
 
 	void (*ping_handler)(struct weston_surface *surface, uint32_t serial);
 
-	int launcher_sock;
+	struct weston_launcher *launcher;
 
 	uint32_t output_id_pool;