[FEATURE] Find image resources in srcset attributes

Extend the HtmlParser to support srcset attributes, which can
be found in img tags or source tags inside of HTML 5 picture
elements, making it fully compatible with responsive images.

Change-Id: If47af154b059c0744c3d660c760fb2cc2d703aeb
Resolves: #71775
Releases: master
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/44901


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>