Data Model

[NoraCodes]

As mentioned in #47 there is not a unified model for mutable/immutable access. This needs to be fixed.

But how, exactly? For instance, when doing an operation requiring &mut access to a Control, should we require &mut access to the UI in use?

[NoraCodes]

My current thinking on this is that we should not require mutable access to the UI handle. I'll submit a PR making the access requirements uniform for the current API soon(TM).

[NoraCodes]

We also need to figure out what callbacks get.

I'm currently of the opinion that we should not give callbacks access to the control they were called from; rather, if they want to change state, there should be some external state and changes to that state should cause changes to the controls.