Feature: pause toast timers when document is hidden

[etcd]

Describe the feature / bug 📝:

This feature adds the ability to pause toasts when the document is hidden. This feature is a UX improvement, because it ensures that toasts don't expire while they're invisible, and all notifications have a chance to be read. Right now, if a toast is generated while the tab is inactive, or if the user switches tabs just after a toast is created, the user can miss the notification.