add "focus" to focused btns with button plugin

possible solution for #12145. 

bit tired tho so might have missed something…

similar to @hnrch02 – except uses focus/blur instead of focusin/focusout. I was going to use the latter, but read firefox doesn't support them (https://developer.mozilla.org/en-US/docs/Web/Events/focusout)… and this appears to work, even though i thought they didn't bubble :eyes: weird.

went back and forth with toggle class, but ultimately thought it might be safer to tie specific events to specific actions

Because of button groups, they toggle via `data-toggle="buttons"`