FTU UI redux (bug 1067462)

Rebase and fixes to get Giovanni's patch at: https://github.com/mozilla-b2g/gaia/pull/24066 to pass tests and land in 2.2
Conceptually, these are radio buttons, right? Meaning, only one can be active at a time. So in essence, they should be marked up as such. Right now, these would be rendered as plain text without any semantic clues as to what they are. If, on the other hand, they are buttons, e. g. each advances the FTU a step further, they should be buttons. That will also add the benefit of immediately making the activation via screen reader working. Bubling up touch events to some parent that will then process stuff will not work with the screen reader.