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.c b/src/compositor.c
index a6610e6..fc69ee2 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -3155,6 +3155,7 @@
 	wl_signal_init(&ec->wake_signal);
 	wl_signal_init(&ec->show_input_panel_signal);
 	wl_signal_init(&ec->hide_input_panel_signal);
+	wl_signal_init(&ec->update_input_panel_signal);
 	wl_signal_init(&ec->seat_created_signal);
 	wl_signal_init(&ec->output_created_signal);
 	ec->launcher_sock = weston_environment_get_fd("WESTON_LAUNCHER_SOCK");