backport client_leave_fullscreen from next branch (Thanks ccount)

include/client.h

@@ -53,6 +53,12 @@ bool client_matches_class_name(Client *client, char *to_class, char *to_title,
  */
 void client_enter_fullscreen(xcb_connection_t *conn, Client *client);
 
+/**
+ * Leaves fullscreen mode for the current client. This is called by toggle_fullscreen.
+ *
+ */
+void client_leave_fullscreen(xcb_connection_t *conn, Client *client);
+
 /**
  * Toggles fullscreen mode for the given client. It updates the data
  * structures and reconfigures (= resizes/moves) the client and its frame to