Focus events and keyboard navigation

[cmawhorter]

I skimmed through #26 and the linked discussions but they seem to be focusing (har har) on displaying different styling based on keyboard vs mouse. #39 might also be relevant

I'd just like to throw out another use --

Currently, there is no good way to know if an element gained focus as a result of keyboard navigation. This makes an entire class of hover ui inaccessible. Like this github component here that you can't tab to: #26

Maybe something purpose-built higher up in focus events would be a better target than here, but I figured I'd mention it.