Simplify hooking to Accessibility Checker loaded event

[mlewand]

Currently there's this inconvenient way to listen for AC readiness, where it could be loaded either:

• synchronously - for released packages
• asynchronously - for dev version

Reason for this is that the dev version uses Require.js for all the deps - which are inlined on release version to get rid of Require.js dependency.

Because of that often our snippets use code like:

var a11ychecker = editor._.a11ychecker;

if ( a11ychecker.exec ) {
	a11yCheckerReady( editor )
} else {
	a11ychecker.once( 'loaded', function() {
		a11yCheckerReady( editor );
	} );
}

To detect whether AC is fully functional yet.

The problem is with this logic fork, what we want to do here is to always have the same way to interact with AC. It's ok if it requires hooking asynchronously, like a11ychecker.once( 'loaded', function() { … }) as long as it does not require this awful if fork.