QNX: Special case z-ordering of the QDesktopWidget window

QNX: Special case z-ordering of the QDesktopWidget window

The assumption that window creation order implies correct initial
z-ordering is broken when dealing with certain window types. In this
commit we special case the QDesktopWidget's window which maybe created
after normal application windows yet still need to be layered below
them.

Without this fix we may accidentaly activate the Desktop window when the
blackberry navigator service sends an event to activate the window
group. That results in broken focus handling.

Change-Id: I42dfde2efb4a0011e37e7bd2e7c5442590606a24
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Kevin Ottens <kevin.ottens.qnx@kdab.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>