macosx: Improve library media source view headers

The library media source views header doesn't look that nice in master for two key reasons:

1. It is never fully properly hidden, causing misalignments between the video view headers and the media source headers, for example
2. It chops off the top of the scroll view below rather unceremoniously
3. It prevents the cool under-titlebar blur when scrolling through items

This MR fixes all of these issues by replacing the old implementation with a new visual effect view overlaid on the scroll views, allowing for the cool effect and fixing alignment issues

After fix:

Screen_Recording_2023-07-19_at_15.28.30

Before fix:

Screen_Recording_2023-07-19_at_15.30.49

Signed-off-by: Claudio Cambra developer@claudiocambra.com