wayland: mucho wip: initial sort-of-working wayland backend

Bar is drawn, though only TOP is supported atm. No
screen/display/output selection is possible yet. Mouse *click* works,
but not setting the cursor.

Lots of debug output, crappy code yada yada.

bar/wayland.h

@@ -0,0 +1,7 @@
+#pragma once
+
+#include "backend.h"
+
+extern const struct backend wayland_backend_iface;
+
+void *bar_backend_wayland_new(void);