Simplifying the stub

I've done some work on this for #872676, following the discussions we had over bugzilla/irc.

Hopefully we can burninate the bug if you're OK with the changes, or at least we can start iterating (for example, replacing the 'fake' menu bar with x-menubar or the appropriate tag). I had a look at the Firefox OS style guide but I didn't copy the entire stylesheet in order to keep things simple and less overwhelming :-)

r? @fwenzel @jlongster