Deprecate `CheckBox` so that all `Element` objects are a subclass of `Text`

[MthwRobinson]

The goal of this issue is to deprecate the CheckBox element type, leaving all remaining Element classes as subclasses of Text. Early in the library the potential for non-text Element types was envisioned, but as the library evolved everything wound u being a subclass of Text.

[hubert-rutkowski85]

Also a bit related (but probably deserves a new issue?), we should move the category field to Element, so that linters will not complain.

[MthwRobinson]

Yeah agree on the category element, @hubert-rutkowski85 could you open a separate issue for that?

[hubert-rutkowski85]

Sure! Created here #3055 and started prototyping it.