Need algorithm for how input events are fired

[garykac]

We need a algorithmic description of how input events are fired.

beforeinput, input

This will need to be coördinated with Input Events and Editing specs: https://w3c.github.io/input-events/
They need to have the hooks necessary for their spec work,

See https://w3c.github.io/uievents/#events-inputevent-event-order

[garykac]

For readability and manageability, this might be better as a separate algorithm from the basic key event algorithm (#142).

Being able to separate this out cleanly might be a good test for how we can have hooks between the algorithms.

[garykac]

Handling this as part of specifying the keyboard/input/composition events. See #142