[Xwt.Mac] Make links in labels have the correct style and be clickable.

Known issue: as described in, for instance, http://www.bdunagan.com/2010/07/10/cocoa-tip-urls-in-nstextfields/ link created in bare NSTextField have several cosmetic issues with font and hand cursor until you click them for the first time. This needs to be addressed separately.

Ref: https://developer.apple.com/library/mac/qa/qa1487/_index.html