Make sure we don't lock the database while a dialog is open.

This can happen when
- the user is picking out a file to save the database as
- a dialog asking the user to save/discard/cancel the current database
  changes is active

It is dangerous to lock the databases while these actions are still
in progress.

Closes #208

src/gui/MainWindow.h

@@ -65,6 +65,7 @@ private Q_SLOTS:
     void applySettingsChanges();
     void trayIconTriggered(QSystemTrayIcon::ActivationReason reason);
     void toggleWindow();
+    void lockDatabasesAfterInactivity();
 
 private:
     static void setShortcut(QAction* action, QKeySequence::StandardKey standard, int fallback = 0);