BlackBerry: Fixed cover mode

When the application is minimized a "thumbnail" version of the current
screen is displayed. The windows were hidden before the navigator could
capture a screenshot, so the cover was just black. This fix avoids that.

In Qt5 "setVisible" doesnt't get called, so it's not a problem there.

Change-Id: I4b657e75afcab03b8b100ac5dd41807e2ce9ef33
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>