FIX: restore b2g agent sheet scrollbars and other mobile tweaks on b2g windows

- new DOM window detected by notification observers
- inject full original b2g agent sheet css
  (e.g. to style buttons as in the FirefoxOS device)

should fix #379 

NOTE: there're some remaining issues related to shell styling, 
because currently any try to inject b2g agent sheet into the homescreen
iframe generates some weird side-effect (homescreen change rotation
instead to lock to portrait) but probably it's more important to restore the style for apps now, and then in a second time try to restore it on the homescreen as well.