Use `HasStrictTraits` for all `Widget` subclasses

[corranwebster]

This is technically not backward compatible, but would improve code quality. The likelihood of breaking things is small, but still this change should be in a major release.

[corranwebster]

Experimented with this, and the main source of breakage is with mocking methods in tests - main needed change in these cases is to mock on the class, not the instance.

It also turns up a number of places where we are using undeclared traits, so probably a first step is to fix those.