Should there be a `Filter::lookup_mut` function?

[museun]

Most of the views I have written reset most of their state per-frame, so setting a new value will get invalidated on the next frame.

But some views could totally support &mut access. This is kind of a hazy distinction and its why I've not implemented it so far.

Would there be many benefits to this function?

Ideally, state goes into the view from its builder and the view returns 'notable' state to the user from its View::update() function (via ui.show() and ui.show_children()).

[museun]

With the addition of Style/ApplicableStyle/StyleKind (And Builder<Style = T> + ViewExt's stuff to compose styles) it would make sense to get something mutably. In general, Views store their ApplicableStyle<SomeStyle> which they get from the builder.

See #45 (comment) for some ideas for this.