Making it possible to distinguish touch/mouse as source of focus

Right now data-focus-source makes it possible to tell keyboard from pointer as the source of focus. We (@halmos and I) would like to be able to further distinguish different types of pointers. The use case is to be able to provide a thicker (or otherwise more visible) focus outline on touch, because you can't see what's under your finger.

In terms of how this gets exposed, I don't think we can change data-focus-source without breaking compatibility. So that would always reflect "pointer". But we can add a second data attribute, say data-pointer-type, that could contain touch/mouse/stylus/catpaw/headbang/etc.

In terms of implementation, just giving it a quick look I believe that https://github.com/medialize/ally.js/blob/master/src/observe/interaction-type.js could expose that distinction (since it knows what event it's reacting to) by incrementing/decrementing modality-specific counters in addition to the pointer one, and then that https://github.com/medialize/ally.js/blob/master/src/style/focus-source.js#L52 could simply make use of that information.

We're open to other approaches of course, this is just what we've thought up. If you're up for this, we'd be happy to file a PR.

Start by reading src/observe/interaction-type.js to see how event modality is tracked, then inspect src/style/focus-source.js around line 52. The change is complete when pointer input remains exposed as data-focus-source="pointer" while a separate attribute distinguishes touch, mouse, stylus, and other pointer types.