[fix bug 1067863] Next/Prev button flicker in onboarding tour

![button-flicker-2](https://cloud.githubusercontent.com/assets/400117/4285388/204141fe-3d86-11e4-9bed-7ab5444060bf.gif)

Using the latest Nightly (35.0a1 2014-09-15) there's some slight flicker in the Next/Prev buttons in the on-boarding tour. Seems to be related to when CSS transforms occur going from one step to the next, and is caused because the buttons are overflowing their parent container visually.

This is likely a bug introduced in Firefox (going to see if I can make a reduced test case and file a bug if so), but we should probably find a workaround in the tour page in the meantime. This change moves the buttons to their own layer with a separate parent element, which fixes the flicker and also feels like solid and more reliable choice.

example URL's to test:

`/firefox/32.0/whatsnew/?oldversion=28.0`
`/firefox/32.0/firstrun/`