libweston-desktop: add signal for title/app-id changes

As discussed on
https://lists.freedesktop.org/archives/wayland-devel/2017-August/034720.html,
it's useful for the shell implementation to know when these change,
for example to relay the information on to taskbars or similar.

To avoid ABI changes or the need to make the weston_desktop_surface
definition public, new functions are introduced for attaching
listeners to these signals.

Signed-off-by: Matt Hoosier <matt.hoosier@gmail.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>

libweston-desktop/libweston-desktop.h

@@ -164,6 +164,9 @@ weston_desktop_surface_set_size(struct weston_desktop_surface *surface,
 				int32_t width, int32_t height);
 void
 weston_desktop_surface_close(struct weston_desktop_surface *surface);
+void
+weston_desktop_surface_add_metadata_listener(struct weston_desktop_surface *surface,
+					     struct wl_listener *listener);
 
 void *
 weston_desktop_surface_get_user_data(struct weston_desktop_surface *surface);

libweston-desktop/surface.c

@@ -64,6 +64,7 @@ struct weston_desktop_surface {
 		char *title;
 		char *app_id;
 		pid_t pid;
+		struct wl_signal metadata_signal;
 	};
 	struct {
 		struct weston_desktop_surface *parent;
@@ -287,6 +288,8 @@ weston_desktop_surface_create(struct weston_desktop *desktop,
 	wl_list_init(&surface->view_list);
 	wl_list_init(&surface->grab_link);
 
+	wl_signal_init(&surface->metadata_signal);
+
 	return surface;
 }
 
@@ -511,6 +514,13 @@ weston_desktop_surface_close(struct weston_desktop_surface *surface)
 					       surface->implementation_data);
 }
 
+WL_EXPORT void
+weston_desktop_surface_add_metadata_listener(struct weston_desktop_surface *surface,
+					     struct wl_listener *listener)
+{
+	wl_signal_add(&surface->metadata_signal, listener);
+}
+
 struct weston_desktop_surface *
 weston_desktop_surface_from_client_link(struct wl_list *link)
 {
@@ -679,28 +689,32 @@ void
 weston_desktop_surface_set_title(struct weston_desktop_surface *surface,
 				 const char *title)
 {
-	char *tmp;
+	char *tmp, *old;
 
 	tmp = strdup(title);
 	if (tmp == NULL)
 		return;
 
-	free(surface->title);
+	old = surface->title;
 	surface->title = tmp;
+	wl_signal_emit(&surface->metadata_signal, surface);
+	free(old);
 }
 
 void
 weston_desktop_surface_set_app_id(struct weston_desktop_surface *surface,
 				  const char *app_id)
 {
-	char *tmp;
+	char *tmp, *old;
 
 	tmp = strdup(app_id);
 	if (tmp == NULL)
 		return;
 
-	free(surface->app_id);
+	old = surface->app_id;
 	surface->app_id = tmp;
+	wl_signal_emit(&surface->metadata_signal, surface);
+	free(old);
 }
 
 void