Bug 851072 - [Keyboard] Add cursor-swiping functionality to the Keyboard

Implement cursor-swiping like in http://sergimansilla.com/blog/swipeselection-in-an-afternoon using the new Virtual Keyboard/IME API (https://wiki.mozilla.org/WebAPI/KeboardIME).

There is a lot of whitespace indented in this commit. I advice to review the actual changes without showing whitespace: https://github.com/mozilla-b2g/gaia/pull/9588/files?w=1