Add reaction callback to custom diffProperty

[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:

This PR changes the diffProperty decorator to add the concept of a reaction callback that is registered by using the decorator at a class method level.

Diff functions are now passed via the second argument of the decorator and are required to pure functions with no side affects.

@diffProperty('foo', diffFunction)
class MyWidget extends WidgetBase {}

When the decorator is used at a method level it will register the function as the reaction callback that is called when one or more of the defined properties are considered changed. The function receives the previous and new properties for the registered keys.

class MyWidget extends WidgetBase {
    @diffProperty('foo', diffFunction)
    protected onFooChanged(previousProperties: any, newProperties: any): void {
        // do something when foo is considered changed by the function passed i.e. `diffFunction`
        // the previous and new foo value is available on the function arguments.
    }
}

This is the function that should be used to apply custom logic based on property changes and replaced the existing onPropertiesChanged catch all decorator.

Note: onPropertiesChanged decorator has been removed as part of these changes.

Resolves #579

[kitsonk]

The code looks good and I like the whole change. Seems worth taking the breaking change on.