When an application requests to go fullscreen, we should make the browser fullscreen too, not just the window within the browser.
r19570 adds a fullscreen toggle to the top bar.
This is as good as it is going to get: we can't trigger fullscreen when an application requests it because we can only request fullscreen in response to user events. (and registering a catch all keyboard / mouse event handler for this is yukky). We can't bring attention to the fullscreen icon either because we the window is about to cover it.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1848