Include ThemeableMixin by default

[agubler]

Type: feature

The following has been addressed in the PR:

• There is a related issue
• All code matches the style guide
• Unit or Functional tests are included in the PR

Description:

Proposal includes ThemeableMixin by default for WidgetBase. Renames the original WidgetBase.ts to Base.ts and adds a new WidgetBase.ts module that imports Base plus other named exports from Base.ts and ThemeableMixin and exports ThemableMixin(Base) as WidgetBase (and also re-exports all the other exports from Base.ts).

This should be backwards compatible with existing usages of WidgetBase even if they have ThemeableMixin applied.

This requires #601 to fix the failing unit test.

Resolves #581

[agubler]

Open to any better suggestions on naming :)

[kitsonk]

Not that I have been a fan of it, but it does seem to be the case here... what if we made WidgetBase and abstract class and named it Widget and make the .render() abstract. That way no one would be able to use it (in TypeScript) without extending it and implementing .render(). I think we might be scared of using Widget because people might actually use it/abuse it. Looking at a few other frameworks, they do use things like Element or Component for that kernel.

[kitsonk]

Ignore me... I didn't notice the full context that WidgetBase is still the main thing that end user will utilise.

[agubler]

The merges for this PR would be "tricky" so closing and will re-raise the change if there is consensus to do so.