Add visual indication of tooltip pinning

Created by: mrnugget

The full context can be found in this Slack conversation.

The short version is: I think it would be really valuable to have any visual indication of tooltip pinning. As as user it's confusing that "clicking" changes the behavior of when a tooltip shows up in a slight and subtle way without giving any indication.

@christinaforney: I assigned this ticket to you, because I'm not sure who else to assign this to. And I'd love to get your opinion on this and the whole Slack thread mentioned above :)