Toolbar and tool buttons style. (#2357)

* Toolbar and tool buttons style.

- Add ability to change toolbar state and tool buttons style via
application settings widget.
- Save/restore toolbar state on app start/finish.
- Resolves #2343.

* Implicitly convert enum to QVariant

src/gui/ApplicationSettingsWidget.h

@@ -55,6 +55,7 @@ private slots:
     void reject();
     void enableAutoSaveOnExit(bool checked);
     void enableSystray(bool checked);
+    void enableToolbarSettings(bool checked);
 
 private:
     QWidget* const m_secWidget;