Enhance update checker

* Reduce initial update check notification to 500 ms to prevent inappropriately stealing focus from user
* Add build flag WITH_XC_UPDATECHECK which defaults to ON
* Update checks are resolved every 7 days instead of every time the application is started
* Better checks for beta builds; ignore snapshots
* Increase test cases

src/gui/ApplicationSettingsWidget.h

@@ -57,6 +57,7 @@ private slots:
     void systrayToggled(bool checked);
     void toolbarSettingsToggled(bool checked);
     void rememberDatabasesToggled(bool checked);
+    void checkUpdatesToggled(bool checked);
 
 private:
     QWidget* const m_secWidget;