[Feature Request] Select / Deselect all favorites button

[LSparky]

Explain in detail what your suggested feature would be used for.
Add a button that allows the user to Select all / Deselect all currently selected favorites.

Describe how it would look if it requires a UI.

Explain why people would want to use it.
It's super annoying to have to select every single checkbox one by one when you want to select all avatars, friends or worlds. Especially when you just want to copy them to your local favorites.

I'll gladly make a PR for this if required.