text: Add support for panels following the cursor

Add input_panel_surface::set_panel to specify input panel surfaces which
are overlaying the application and are following the input cursor.

Signed-off-by: Jan Arne Petersen <jpetersen@openismus.com>
diff --git a/src/compositor.h b/src/compositor.h
index 7da6c48..93a82b2 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -319,6 +319,7 @@
 
 	struct wl_signal show_input_panel_signal;
 	struct wl_signal hide_input_panel_signal;
+	struct wl_signal update_input_panel_signal;
 
 	struct wl_signal seat_created_signal;
 	struct wl_signal output_created_signal;