Carousel indicators for IE8-9

Fixes #10903 and #10457: IE8-9 don't support events on elements without `background-color`, so we have to hack it with some `\9` and `rgba()`. As the code comment details, there's no way around this right now given that IE8 doesn't support events on an element with a fully transparent `filter` either.

/cc @cvrebert @clouder @np28