Feature: Apply a custom class to a clip / holder

[negue]

If you have a screen with custom styles and you want to have specific clips to be with a special border or something, it would be nice to apply a custom class to the item / holder.

In a 2nd step those classes could be extended with "triggered-with-TYPE".

In a 3rd step, something like twitch variables could be added like "triggered-by-USER" or "cheer" or "sub" or or or

2nd / 3rd would be automatic labels nothing a user would add, BUT if those were added, they could be styled if needed.

Could be useful with #142