Input Events Level {1,2}

[javan]

This PR improves the way Trix records input—particularly on mobile—by adding support for Level 2 InputEvents, which are currently implemented in Safari and Chrome, and in their companion webview implementations on iOS and Android. In all other browsers, Trix will continue to work exactly as it does today. Here's how:

First, Trix tests for InputEvent support:

trix/src/trix/index.coffee.erb

Lines 21 to 24 in 650ee9a

	supportsInputEvents: do ->
	typeof InputEvent isnt "undefined" and
	"inputType" of InputEvent.prototype and
	"data" of InputEvent.prototype

Then, when a <trix-editor> initializes, its editor controller installs one of two input controllers:

trix/src/trix/controllers/editor_controller.coffee

Lines 24 to 26 in 650ee9a

	@inputController = new Trix["Level#{Trix.config.input.getLevel()}InputController"](@editorElement)
	@inputController.delegate = this
	@inputController.responder = @composition

1. Trix.Level2InputController if input events are supported
2. Trix.Level0InputController, which is equivalent to the current Trix.InputController, if input events are not supported

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Fixes #575
Fixes #574
Fixes #520
Fixes #494
Fixes #396
Fixes #315
Fixes #178
Closes #564