Bug 906598 - Move Keyboard settings into app itself

This brings back the settings screen, but now as a child of the keyboard application itself. I think we should not rely on global settings for this in the future but rather go to asyncStorage but that'll breaking customization scripts, FTU, language settings, at the moment.

I've moved the en-US localization strings out of settings and into this app. Guess that's enough?

This PR includes RudyLu/3rdParty_keyboard_support_commit2, and two patches of mine to allow it to work in FF (yay, easier development). 

1. Land RudyLu/3rdParty_keyboard_support_commit2
2. Land this