LanguageClient: Add LanguageClientManager::clientAdded() signal

For consumption by interested parties.

Change-Id: I0cbecb4ce99cdd208fc3ff019d4f0ea672dcf752
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/languageclient/languageclientmanager.h

@@ -103,6 +103,7 @@ public:
     static void showInspector();
 
 signals:
+    void clientAdded(Client *client);
     void clientRemoved(Client *client);
     void shutdownFinished();