Fix a missing quote mark in Javascript + various other fixes -  accessibility slider control [Pull #1292]

Hi @johndyer and @rylan,

Rylan - thanks for your work on accessibility fixes - particularly on making hidden control bars work with screen readers. I got a colleague to [test our two progress bar enhancements](https://docs.google.com/document/d/1d1QKaKfOR6xDWw4UQIH7AN6GuxE8cpIUvYNoNTmAcKM/edit). As you discovered, your fixes came out better (though my colleague didn't test with NVDA or experience the specific problem that you found).

John and Rylan - I've fixed a missing quote mark in the commits for pull #1292, and some other improvements. I meant to get these to Rylan earlier, so that he could review - time has slipped by...

I hope this pull request helps. Best wishes,

Nick


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https://github.com/IET-OU/mediaelement/commit/9359d22963f9b324d8e17f0fe47fb5462223b4f0

* Improve CSS style for ".mejs-offscreen" - prefer "top" for RTL languages.
* Make the text "Use Left/Right Arrow keys..." an option - so customized players can modify where and how they present this help text;
* Clarify JS comments - "Accessibility for slider" in place of "Ally for slider";
* Ensure "updateSlider()" Javascript function is called - to add WAI-ARIA attributes - role, aria-now, aria-min ...;
* src/js/mep-player.js:282 - ensure "videoPlayerTitle" applies to audio players as well as video players;