Stop automatically deleting missing files from a playlist

This will stop automatically deleting missing files from a playlist when they're not found in the music store (via mediascanner2). This will avoid deleting songs from a playlist while mediascanner2 is doing its thing such as when syncing via UBsync, or manipulating files in the Music directory.

Instead, there's a warning strip shown in the playlist song list view and has an option to open a dialog to show the affected songs/files and offer an option to remove them from the playlist.