Accessibility integration

The following patch is a combination of the Composer patch from #2454351, and some bespoke code designed to add accessibility features to Owl Carousel without modifying the original library.

The accessibility code does the following:

* Defines WAI-ARIA attributes for the entire carousel widget
* Make the carousel and its individual panes focusable
* Adds keyboard events to the left/right/up/down keys to move the carousel when the carousel is focused
* Adds a description when the user focuses into the carousel
* If the carousel is set to stop on hover, stops playing on focus, and starts playing again when focus is lost
* Triggers an update every time the carousel rotates, in order for screen readers to read out the current content of the carousel

I've had a functional methodology in mind when designing the accessibility code, and have chosen to implement it using the mindset that it's a "glue" layer on top of Owl Carousel, instead of modifying the original code. This sort of approach might allow us to add extra functionality in the same sort of way, allowing us to do things like configurable thumbnails, custom behaviour, etc.

Before people start running off to use this in production, keep in mind that it has not yet undergone formal accessibility review, so it's probably not ready for use in situations while it really matters. Also, while WAI-ARIA attributes have been implemented, keep in mind that browser support for them (unfortunately) remains variable.
Furthermore, I've had some problems with it being a bit crashy on Chrome (to my endless frustration), but it seems to behave pretty well in other browsers, particularly Firefox.
Finally, I get the feeling that this issue might open up more tins of worms as far as extending Owl Carousel goes.

Let me know what you think; this goes for Owl Carousel 2.x users too.