qt: "Media Library" context menu action is no longer checkable
... and its title is properly adjusted such that
- If already in player view, the title is "Show main view".
- If already in main view, the title is "Show player view".
Player view is currently considered another page, therefore toggling does not make sense here (also considering the navigation history). This button now always pushes into the history.
Also see discussion in !3732 (merged).
Request review @chub.