Adds support for external image urls

[maechler]

I finally managed to create a pull request. This pull request fixes #1439.

What I did is add a check whether we are dealing with an already fully qualified URI or not (if (isset($parsedSrc['host']))). The else part is the function as it was before. This implementation is inspired by what TYPO3 Core does: https://github.com/TYPO3-CMS/extbase/blob/96fffc2b245dc29a78bcb06a64cdb9d138f169c9/Classes/Service/ImageService.php#L84-L103

This of course does not add full support for FAL (e.g. no image processing), but it enables fully qualified URIs to be used in frontend and backend.