WebAPI/InputMethod API with hardware keyboard

Meta bug: bug 929365.

Idea

The previous iteration of InputMethod API (See WebAPI/KeboardIME) enables the keyboard apps to interact with the targeted input field. We would like it to handle the connected hardware keyboard as well.

Proposal

This proposal is based on WebAPI/BrowserAPI/KeyboardEvent.

1. The blue lines are the current behavior. [line (1)]
2. This proposal is about adding keyboard app into the chain right before the event target (the active app).
3. In mozbrowser*before*keyXXX phase, Gecko dispatches the key events to keyboard app prior to deliver it to the event target if and only if keyboard app is active. [line (2)]
4. If keyboard app wants to handle certain keys, it should cancel the events by calling "preventDefault" (or something else, we'll talk about it later).
5. If keyboard app doesn't cancel the events, Gecko will then dispatch it to the event target as usual. [line (3)]
6. If keyboard app cancels the events, it should be regarded the same as cancelled by the event target and follow the same rules defined in WebAPI/BrowserAPI/KeyboardEvent.