feat(popover): Single mode popover implemented using broadcasts

This is a third way of implementing this feature. I already sent 2 more pull requests which do it in other ways. I think this one is the cleanest way of doing it.

It preserves the convenience of configuring popovers using tooltipProvider.options({popoverMode:'single'}), and also keeps the code properly abstracted.

Let me know what you think.

Cheers,
Varun.