`Add to playlist` dialog not unique.
It is possible to open multiple add to playlist dialogs at the same time.
Steps to reproduce:
- Select a large set of songs/albums/artists
- Click the Add to playlist button multiple times
- Multiple dialogs open
- Closing these dialogs sometimes crashes
- Only 1 dialog open at a time.
This is probably because the dialog does not immediately open, it is delayed by the async loading of the songslist to add. The loading should probably not hold up the dialog opening.