Explicit event API

[Tyriar]

This PR exposes the new explicit events API that will replace the old node-like EventEmitter. I've converted all internal events over to the new EventEmitter2, the only usage of EventEmitter remaining is within Terminal to refire the events so we don't break compatibility until v4.

Some of the events have changes a little or been removed, see below:

• blur -> Removed as you can just listen to the DOM event on Terminal.textarea
• focus -> Removed as you can just listen to the DOM event on Terminal.textarea
• linefeed -> onLineFeed
• selection -> onSelectionChange
• data -> onData
• key -> onKey
• keypress -> Removed in favor of a single onKey
• keydown -> Removed in favor of a single onKey
• refresh -> onRender
• resize -> onResize
• scroll -> onScroll
• title -> onTitleChange

New events being added:

• onCursorMove, this was already being used by the search addon and also within VS Code

If anyone sees events they rely upon removed please let me know how you're using it as on is now deprecated and will be removed soon.

Fixes #1505

[vincentwoo]

If .off is deprecated, is there a new way to remove event listeners now?

[Tyriar]

@vincentwoo onData, etc. return an IDisposable. Hold on to that and call .dispose() on it to remove the listener.

[vincentwoo]

oh ho ho ho, fancy

[Tyriar]

😎