Allow ui_state attribute to be used with arbitrary associated struct name

[Vollbrecht]

Is your feature request related to a problem? Please describe.
Its not documented and non intuitive that the ui_state attribute can only be used with something that is exactly called RustState

Describe the solution you'd like
Allow for arbitrary struct naming of a struct annotated with ui_state

Describe alternatives you've considered
Warn the user if he uses the ui_state attribute on a struct that is not exactly RustState and document this implicit behavior.

[fzyzcjy]

Totally agree, we should support that! (Originally it was more like a quick demo so was quite minimal) Feel free to PR, alternatively I will work on it later (may not guarantee to be within days since this is related to experimental feature).

Btw possibly related: #2307