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:

Convert widget-core to use TS2.2 class mixins instead of dojo/compose

There are a couple of interesting changes included

1. The baseTheme property for the Themeable mixin is set via a class decorator (@theme exported from Themeable)
2. The “result” from using the class Mixins requires a generic to define the Widget’s properties class MyWidget extends ThemeableMixin(WidgetBase)<MyWidgetProperties> and excluding this does actually provide a pretty obscure error No base constructor has the specified number of type arguments. along with a few others.
3. I’ve removed the default export for all base, mixin and main classes in place of named exports (This is personal preference, so we can debate this potentially)
4. The constructor argument for any widget is now just properties, the WidgetOptions class/interface has been removed.
5. Allowed pascal casing for variables, mainly because the result can be a class in cases like const Projector = ProjectorMixin(Button);

Items to follow up on:

1. A generic solution for decorating the prototype with attributes, eg. provide an object bag from widget base that attributes get set to and then copied from for usage.
2. Decorators to add functions to a list used by widget base for "property specific diffs" and "render decorators". This will enable us to move away from strict regular expressions.
3. Need to check the order that "decorator" functions get applied, should work back up the inheritance chain.
4. Type doc corrected/added where appropriate

Resolves #327