Bug 1039519 - Add a show/hide rocketbar transition. r=daleharvey,kgrandon

This is the preliminary work for showing/hiding the rocketbar in response
to user scrolling. There is still some follow-up, which will be addressed
in subsequent bugs.

Problems:
- Overscroll on non-scrollable pages is broken (Gfx bug 1040226)
- All pages with navigation can be scrolled a little (needs bug 757859)
- Overscroll should be the page background colour, isn't always
This is needed to scale the text at a different rate to its container