improve Hide from Library workflow

[ronso0]

When attempting to hide multiple tracks from the library one or more tracks might be in a playlist, regular or history playlist. A dialog pops up saying which playlists are affected, but not which tracks are affected.
With many tracks selected, finding the affected tracks is cumbersome for various reasons: first of all you'd need to memorize the playlists, then the Hide operation has to be canceled and the selection be cleared, then you'd have to go through the tracks, find those which are affected, decide what to do with them (hide, ignore?), try to restore previous selection....waaaaah

First improvement would be to offer expanding a list of the tracks in question. This might be a simple track list or even some kind of tree view, and allows the user to modify the track selection. The dialog window needs to be non-modal for that to work.
TrackXyZ --- playlist1
           - playlist2
           - playlist3
TrackAlk --- playlist1
           - playlist4

As a second step, that list might have checkboxes for which tracks to keep in which playlist(s).
Track checked: keep track
Playlist checked: keep track (greyed out)
Track checkbox override playlist checkboxes.
[ ]TrackXyZ ---[ ] playlist1
              -[x] playlist2
              -[ ] playlist3
[x]TrackAlk ---[x] playlist1
              -[x] playlist4

Related:
Remove from disk menu item
https://github.com/mixxxdj/mixxx/pull/1737

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/9914