Refactor cage into separate source files

This makes Cage much easier to maintain. Not only is it easier where to
look and to maintain a mental model of the code, there is also more
encapsulation, better abstractions and better extendability.

output.h

@@ -0,0 +1,19 @@
+#ifndef CG_OUTPUT_H
+#define CG_OUTPUT_H
+
+#include <wayland-server.h>
+#include <wlr/types/wlr_output.h>
+
+#include "server.h"
+
+struct cg_output {
+	struct cg_server *server;
+	struct wlr_output *wlr_output;
+
+	struct wl_listener frame;
+	struct wl_listener destroy;
+};
+
+void handle_new_output(struct wl_listener *listener, void *data);
+
+#endif