feat(api): add responsive API for img elements

[ThomasBurleson]

• add ImgSrcsetDirective
  • to inject elements to support responsive images
  • Inject a element for every srcset. in the HTML markup of an element contained in a elemen
  • support usages without <picture> parents
• add responsive API to img.src: src.md, src.lt-lg, src.gt-xs, etc.
• repackage API classes to easily distinguish flexbox APIs and extended responsive APIs

Closes #366, Fixes #81, Fixes #376.

[ThomasBurleson]

This PR uses the work (and changes and approvals) from PR #366. It adds:

• more unit tests
• support for responsive img.src APIs
• repackaging of API classes

@benbraou - please confirm the use of your commit here in this PR. Can you also review the new additions img-src.ts and img-src.spec.ts ?
@andrewseguin - can you review these two files also? No need to review the other files as Miles and Ben and I reviewed those in PR #366.

Once this all looks good, let's merge ASAP as this is the LAST merge before we release Beta.9.

[andrewseguin]

img-src.ts and img-src.spec.ts look alright to me, from a naive review

[benbraou]

I am starting the review

[googlebot]

CLAs look good, thanks!

[ThomasBurleson]

@benbraou - based on your feedback and our continued efforts with testing on ImgSrcsetDirective, I have removed that functionality from this PR.

We will keep PR #366 open for more tests, commits, and stabilizations.

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