List widgets are currently used as playlists in source properties,
but only contain the file paths and no other identifying information.
This can lead to files being added multiple times, so when changes to
list order occurs, plugins cannot uniquely identify which duplicate
item was actually changed (because they're only identified by the path).
By adding a UUID to the user data role of a list item, an additional
unique information is added that allows plugins to de-duplicate list
items.
