Add field to filter existing playlists in “add to playlist…” dialog

Also:

• Fix the duplicate track add confirmation margins
• Don’t close the duplicate track add confirmation if clicking cancel
• Improve header of add to playlist dialog, making the dialog more compact

Fix #974 (closed) (and maybe reopen #429?)

Other improvements idea: https://pad.funkwhale.audio/C00gMaUMQlGufpzJY6uKQQ

Before:

After: