Pull #1292, Fixing a missing quote mark - Accessibility slider control
Pull #1292, Various fixes to accessibility enhancements to the slider

* 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 "[sic] 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;
Merge pull request #1342 from IET-OU/allyProgress

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