improve mixpanel usage statistics (#2238)

Other changes:
- Always display first start dialog if privacy options are unset (e.g. if the user closed GPT4All without selecting them)
- LocalDocs scanQueue is now always deferred
- Fix a potential crash in magic_match
- LocalDocs indexing is now started after the first start dialog is dismissed so usage stats are included

Signed-off-by: Jared Van Bortel <jared@nomic.ai>

gpt4all-chat/chatllm.h

@@ -123,9 +123,7 @@ Q_SIGNALS:
     void modelLoadingWarning(const QString &warning);
     void responseChanged(const QString &response);
     void promptProcessing();
-    void responseStopped();
-    void sendStartup();
-    void sendModelLoaded();
+    void responseStopped(qint64 promptResponseMs);
     void generatedNameChanged(const QString &name);
     void stateChanged();
     void threadStarted();