Bug 1074332 - Utility tray improved transtion r=mhenretty

We were using .utility-tray class for making the utiltyTray visible, but now that class is only added when we pass the statusbar (so we can hide the app statusbar). We need something that makes the utilityTray visible from the begining of the dragging gesture. I'll try to rethink if we can merge that need with the current 'hidden' class instead adding a new one.