Prevent prompt spamming

[paulrouget]

We have a prompt system in place. We need to give the option to the user to dismiss further prompt to avoid spamming.

That is true for content dialogs (window.alert()), but also true for things like the devtools permission prompt.

[Manishearth]

In general permissions-requiring APIs are typically designed such that they must have user activation to be triggered, so it's hard to do this with permissions APIs. But still worth having this filter.