feat(ScopeAware): Introduce ScopeAware abstract class.

[rkirov]

Components that implement the ScopeAware scope setter, will get scope
set during compilation.

Currently, there is little incentive to use this, because Scope can be
injected in the Components. However, after PR/1269 Scope would not be
injectable and this would be the only way for components to obtain
scope.

Introducing this API earlier allows for clients to start using it and
not break when PR/1269 lands.

[mhevery]

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[vicb]

LGTM

[vicb]

As #1364 add circular dep detection, it would be easy now to wrap the component creation in a try-catch and display a warning if a circular dependency is detected.
I'm not sure if we can reliably detect whether the Scope is injected - we would have to introspect the arguments which is not possible when the code is compiled to JS plus it could be a transitive dependency.
However a (transitive) dep on Scope is probably the only possible source of exceptions and we can display a message accordingly, referring to ScopeAware.

[rkirov]

Yes, as @vicb said the circular detection is the right way to do this because otherwise we would miss transitive deps (Component needs Foo, Foo needs Scope).

@vicb That's a nice idea, once #1364 you can modify it to mention "ScopeAware" when Scope is detected as part of the repeated loop. That would be much more user-friendly.

I am going to go ahead and merge this commit, as it doesn't introduce the Scope injection problem, but only the solution - ScopeAware, so that I can start moving clients to it.