Call busy indicator from C++

Given the number of identical calls, most pool retrieval functions have
been also aggregated in a single function, postDownload.

src/mainwindow.h

@@ -93,6 +93,7 @@ private:
     void setupDockWidgets();
     void setupConnections();
     void clearModels();
+    void postDownload(const QStringList& tags = QStringList(), bool relatedTags = false);
 
 private Q_SLOTS:
     void connectToBoard();