LanguageClient: Add signal to inform about WorkDoneProgressEnd

... and make use of it in the clangd client to be able tell when
background indexing has finished.

Change-Id: I0f3c6f9646fd66ababd08c12b2f347da5f1a3729
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/languageclient/progressmanager.h

@@ -49,6 +49,8 @@ public:
     void setTitleForToken(const LanguageServerProtocol::ProgressToken &token,
                           const QString &message);
 
+    static bool isProgressEndMessage(const LanguageServerProtocol::ProgressParams &params);
+
 private:
     void beginProgress(const LanguageServerProtocol::ProgressToken &token,
                        const LanguageServerProtocol::WorkDoneProgressBegin &begin);