[sailfish-browser] Attach browser cover to the content window. Contributes to JB#29295
[sailfish-browser] Attach browser cover to the content window. Contributes to JB#29295

Lipstick expects the cover related properties set on the
"main window".

sailfishos/sailfish-browser,sailfishos/sailfish-browser,sailfishos/sailfish-browser,sailfishos/sailfish-browser,sailfishos/sailfish-browser