Proposal: move Button alias for Event to traitsui.api

Traits currently has an `Event` subclass called `Button`. It would make some sense to move that subclass from Traits to TraitsUI. This would need to be done gradually, to avoid gratuitous breakage. A first step would be to make `Button` available in `traitsui.api` (leaving the implementation in Traits for now) and encourage people to import from `traitsui.api` instead of `traits.api`.