[Qt] Debug window: replace "Build date" with "Datadir"

The build date does only makes sense for custom/self-compiled bitcoin-core versions because we are using static build-dates for our deterministic release builds.
Having a quick option to get the current datadir is much more valuable for debug purposes.

src/qt/clientmodel.h

@@ -76,6 +76,7 @@ public:
     bool isReleaseVersion() const;
     QString clientName() const;
     QString formatClientStartupTime() const;
+    QString dataDir() const;
 
 private:
     OptionsModel *optionsModel;