API request: Adding WebEngine.userAgent

[Peter Zhelezniakov]

> From: Werner Lehmann <lehmann at media-interactive.de>
Makes sense. I suggest to use a different defaultValue though ;-)

Yep, thanks for noticing. I think I'll remove this line.

The default value is system dependent. E.g. on Linux it is

Mozilla/5.0 (Linux x86_64) AppleWebKit/537.2 (KHTML, like Gecko)
JavaFX/8.0 Safari/537.2

Compare that to Chrome's ID:

Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/536.5 (KHTML, like Gecko)
Chrome/19.0.1084.52 Safari/536.5

or Safari's on Mac:

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_6_8) AppleWebKit/534.57.2
(KHTML, like Gecko) Version/5.1.7 Safari/534.57.2

So we follow the pattern used by other Webkit-based browsers, report
proper version of Webkit, and proudly mention JavaFX at the same time ;)

-- 
Peter