MediaPlayListProxyModel: only enqueue after the current track when shuffled and only after the current album when in album shuffle mode
Reason for the change
Typically if a user wants to enqueue a track, they also want to hear the track at some point.
Also fixes an issue where the enqueued track could sometimes enqueue at the index of the currently playing track, causing the current track to change.
Test plan
Amended the autotest to ensure the tracks are always added after the current track