Make Videogular controls more accessible

This change set makes a number of improvements to keyboard accessibility for Videogular controls. In particular:

* The play, mute and full-screen buttons now use `<button>` tags instead of `<div>`s, allowing the user to focus on them with the tab key and 'click' them with the space bar. It also better reflects the elements' roles, which should improve screen reader compatibility. Under the default theme, a border appears around a focussed button.

* The scrub bar can now be focussed with tab key. When it is in focus, the left and right cursor keys seek backwards and forwards in the video. Under the default theme, an extra line appears at the current time when the scrub bar is in focus.

* When the mute button is in focus, the volume bar becomes visible and the up and down cursor keys can be used to change the volume. (This is in addition to the default functionality of the button, which toggles mute when space is pressed.)

During the work, we discovered that the autohide option should not be used when the controls must be accessible, as it prevents the controls from being focussed.