Automatic cleanup of listeners on other components

[heff]

Implemented half of #1544. Now instead of using:

player.on('some-event', vjs.bind(this, fn));

You can use:

this.on(player, 'some-event', fn);

Using the second form will automatically clean up the listener whenever either the player or the component is disposed. And 'player' can be replaced with any other component.

When implementing this in existing components I found a few places we weren't removing listeners on dispose, and they should be fixed now with this.

UPDATED to include other elements, e.g.

this.on(document, 'some-event', fn);

[heff]

I added the option to attach listeners to other elements. component.on(someElement, 'some-event', fn);

With regular elements there's potential here to create a memory leak, because if you remove an element from the DOM without removing the listener, there will still be a reference to the element until the component is disposed. So originally @mmcc and I discussed an API like component.onDoc() specifically, because the document is permanent. But thinking through it, we need to be good about cleaning up listeners either way, so allowing on() to be used on elements doesn't make that any more complicated. And with elements you can still use vjs.trigger(someElement, 'dispose') to clean up these listeners before removing the element from the DOM.

I'd love to get a second set of eyes on this if anyone's up for it.

[hacfi]

👍 very nice!