JBIDE-15742 Make Content Assistant Additional Info window to support URLs.

Adds a possibility to show an external (http- and https-like) URLs in Completion Proposal Additional Information window.
Because of that fact that not all web pages are shown correcty in Eclipse's internal Web browser, the URLs are opened always in
external browser.
JBIDE-15713 Advanced HTML5 Properties view

Initial commit.
JBIDE-16019 Reorganize JST component