Add helper for storing/retrieving the accessible name of a QAction

Qt only supports setting accessible names for widgets. Therefore, we
change to existing helpers getAccessibleName/getAccessibleDescription
to accept only QWidget instead of QObject. Additionally, we add
setAccessibleName/getAccessibleName helpers which store the accessible
name of a QAction as custom property of the action.

• GnuPG-bug-id: T6046