compositor: Move input devices to their own event loop

First step towards only handling input at repaint time.
diff --git a/src/compositor.h b/src/compositor.h
index 6f46249..b94731b 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -184,6 +184,9 @@
 
 	struct weston_shell *shell;
 
+	struct wl_event_loop *input_loop;
+	struct wl_event_source *input_loop_source;
+
 	/* There can be more than one, but not right now... */
 	struct wl_input_device *input_device;