Current implementation was not working correctly
for quicknanobrowser when entering and leaving
fullscreen video, in cases where the browser itself
was already fullscreen. Use ExitFullScreen
to leave fullscreen in demo code. Use onFullScreenRequested
handler to track if fullscreen mode is on.

Change-Id: I022d5e830b189897d34a9a8747381d041101c692
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>