Accessibility improvements for the player

This is added functionality to make the player more accessible for people with disabilities. There are several changes which are broken up into logical commits. The additional functionality is:

1. Adding support for writing "aria-*" and the "role" attributes.
2. Showing the keyboard focus at all times
3. Reordering the creation order, and thus insertion order of the UI elements, making the tab order more logical
4. Adding a hidden label to the big play button for screen reader users
5. Changing the button text when it toggles, e.g. play/pause, mute/unmute, full screen/non-full screen
6. Optionally allow the player controls to be visible at all times
7. Add additional information about the volume bar for screen reader users
8. Add additional information about the progress bar, current time, and time remainging for screen reader users
9. Adjust the size of the captions proportionally when switching to full screen mode
10. Making the caption, subtitle, and chapter buttons more usable by screen reader users