Make sound selector work with large sets of sounds

[ffont]

Currently we have a "sound selector" widget which we use in the pack edit page and in the sound edit page so specify the sounds of a pack and the sources of a sound respectively. This works well except when a pack has many sounds or a sound has many sources. Then the loading is very slow as the widget is not paginated. We should think of a way to improve user experience when editing large packs or sounds with many sources. Most likely we can paginate the widget, but this needs to be done carefully as the widget's logic is already quite complex.

Also in #1718 a new feature is suggested for the sound selector to add the option of sorting the selection. Maybe both improvements could be addressed in one go.