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).

https://bugzilla.mozilla.org/show_bug.cgi?id=851072