heavy placeholder refactoring, in order to support autoplay in p-js.
player.css contains the style of the container, instead of setting it in js.
SHARED_VIDEO_ELEMENT_ACTIVATED - it's certain the video element can play.
autoPlay and canAutoPlay, the latter set to true once the SHARED_VIDEO_ELEMENT_ACTIVATED is true, and after a click.
when one video plays, show placeholders on the others.
allow auto play once a video element has been activated.
remove modules in order to reinitialize a player.
use moduleId on objects created outside the module manager.
getPlayer returns the first player if uri is not defined as an argument.
placeholder cleans itself up and replaces the contents with the initial html.