[css-env-1] values for hiding fullscreen browser UI

[grorg]

Following on from #2871, the browser/host might show some UI when in fullscreen, but hide that UI after a while. For example, a video site that wants to show media in fullscreen with a custom UI will need to use #2871 to avoid the browser button to exit fullscreen. However, the browser doesn't want to constantly show the UI (distracting the user from the media).

• fullscreen-auto-hide-duration - a <time> value that describes how long the browser will take to fade out its UI
• fullscreen-auto-hide-delay - a <time> value that describes how long before the browser begins the fade