Include WebKit version on about page

[tresf]

JavaFX Provides the following:

web.getEngine().getUserAgent()

Which may return something like:

[...] JavaFX/8.0 Safari/537.44

Which can be cross-referenced with:
https://en.wikipedia.org/wiki/Safari_version_history#Safari_6

To see that 537 is equivalent to Safari 6.1.6, which will help with websites like https://developer.mozilla.org/ to find out if a particular feature is supported.

Note: Apple may move or remove this feature, see #552