Indicator.css: Add a background pulse to non-muted mic

[danirabbit]

A little more eye-catching and "active" feeling

[cassidyjames]

Hm, I'm not sure. I don't know if I want system UI pulsing for more than a moment: it means the GPU is constantly doing work, invalidating the pixel cache, and not able to go into a lower power state. Though to be honest I'd have to ask a GTK expert what the real draw of that would be (or do some testing).

I also don't know that I like the design; it doesn't work on a blue background at all and the oval doesn't quite fit in with the other indicators to me. Would it make sense to just have the mic itself constantly blue (so we get the shadow protecting the contrast a bit), kind of like how we have the muted speaker notification orange/yellow? Or even just pulsing less constantly, like once every few seconds?

[jeremypw]

Converting to draft as there are unresolved design issues.