Support loading images from inline data-URIs

[Michael Strauß]

I would like to propose adding support for loading images from inline
data-URIs, as commonly supported by web browsers.
This is a low-impact addition to the image loading infrastructure as
seen in this draft PR: https://github.com/openjdk/jfx/pull/508

As a follow-on feature, I'd also like to support loading stylesheets
from data-URIs.
With both features implemented, developers have more flexibility in
working with stylesheets.

For example, it'd be possible to dynamically generate and add
stylesheets to applications without deploying temporary files on disk.
Another scenario would be to include small theme-specific icons in
their respective stylesheets, which can simplify deployment.

Any thoughts?