b2g (and gecko_mobile and gecko_tablet) 

Adds support (mgwt.os values) for gecko_mobile, gecko_tablet and b2g (just a variant of gecko_mobile but 
has to be detected looking at the user-agent). 
Fixes some css related issues on gecko.

The Showcase seems to work quite well:  http://mgwt2g.appspot.com/

In the showcase remain issues with the height of the selector on the GroupList we had no chance to fix and 
some sizing problems on Forms and Elements, probably due to  css.