[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.cpp

@@ -193,6 +193,11 @@ QString ClientModel::formatClientStartupTime() const
     return QDateTime::fromTime_t(nClientStartupTime).toString();
 }
 
+QString ClientModel::dataDir() const
+{
+    return QString::fromStdString(GetDataDir().string());
+}
+
 void ClientModel::updateBanlist()
 {
     banTableModel->refresh();