visibility state and notifications

[dominictarr]

ideas:

emit focus and blur events so that the page knows when it's on the screen.

Also, decouple the tab list. this should be quite different on mobile. it should be a separate module, with a default probably.

tabs should also have nicer titles - use mutation observer to watch the title attribute? and favicon?

it would also be a good idea to have a notification state, so we can see whether the content of that tab wants to update? or do this via favicon?