Add "Blocksdir" to Debug window

To get the current blocksdir is valuable for debug purposes after
merging #12653.

src/qt/clientmodel.cpp

@@ -177,6 +177,11 @@ QString ClientModel::dataDir() const
     return GUIUtil::boostPathToQString(GetDataDir());
 }
 
+QString ClientModel::blocksDir() const
+{
+    return GUIUtil::boostPathToQString(GetBlocksDir());
+}
+
 void ClientModel::updateBanlist()
 {
     banTableModel->refresh();