LanguageClient: Add the possibility to override progress messages

... and make use of it in the clangd client.
We want the progress bar for background indexing to have a better title
than the plain (and non-translated) "indexing" that the clangd server
sends.

Change-Id: Ib75eac370e7c22f0c5bd477f4a4c423283b27e1f
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/languageclient/client.h

@@ -182,6 +182,8 @@ signals:
 
 protected:
     void setError(const QString &message);
+    void setProgressTitleForToken(const LanguageServerProtocol::ProgressToken &token,
+                                  const QString &message);
     void handleMessage(const LanguageServerProtocol::BaseMessage &message);
 
 private: