Bug 908113 - Stop fading in header elements

This patch changes the way we do animations in the header in Gaia.

1. All header elements now get the background color instantly instead of via a 0.2s animation
2. The back/cancel button keeps it active color when clicked for 1s to allow the cancel animation to play but not discarding the back animation
3. All other buttons will remain their active color for 0.1s so the user feels he actually clicked something
4. Back / cancel buttons are now 7rem instead of 5rem to improve clickability