A new class that derives from `QMenu` has been introduced. Menus of this
`NonAutodismissibleMenu` type will stay visible when a _checkable_
action is triggered.
This is convenient in menus that feature a series of check boxes that
toggle visibility of third components (e.g. the **List Columns** menu),
allowing the user to toggle several actions at once.
For now, the new type is used in the top menu bar.
