qt/qml: Update the 'Media Library' toggle behavior

This MR updates the 'Media Library' toggle behavior, by adding a 'Media Library' checkable entry in the menu bar. This makes the player view accessible at any time, even when a media is not playing. That's actually the behavior we have on 3.x with the MediaLibrary / Playqueue ancestor.

This MR also updates the TopBar QML to fit our design choices surrounding classic. In particular only displaying the menu bar and nothing else when the player is pinned.

Overall, it's the closest we've been from a 3.x legacy UI ever, which should reassure the more conservatory types among us.

Fixes #28174 (closed)

NOTE: The media_library.svg is a placeholder for now.