dbhub: Keep track of the cloned databases

This adds a local database to keep track of all the cloned databases.
For now we only use this information to prevent exactly the same
database being downloaded twice.

src/PreferencesDialog.cpp

@@ -4,6 +4,7 @@
 #include "Settings.h"
 #include "Application.h"
 #include "MainWindow.h"
+#include "RemoteDatabase.h"
 
 #include <QDir>
 #include <QColorDialog>