Show in the top bar when a newer version is available

At start and after switching server, the game asks the server for the newest
MusicPlayer release and shows "Version X is available" in the top bar when it
is newer than this one.

tests/integration/test_scene_split.gd

@@ -29,7 +29,7 @@ const LOCKED_BY_SYNC: Array[String] = [
 
 const TOP_BAR_CHILDREN: Array[String] = [
 	"TopPanelContainer", "Title", "SyncButton", "SettingsButton", "ShortcutsButton",
-	"DebugLabel",
+	"DebugLabel", "UpdateLabel",
 	"AboutButton", "QRButton", "QuizTab", "StatisticsTab",
 ]