BlackBerry: show dialogs full screen

Except for message box, which will have their optimal geometry calculated at
runtime.

This fix is not necessary on Qt5

Change-Id: Ida4743acdceb5424ab93aa3a32c78db61dd6795c
Reviewed-by: Nicolas Arnaud-Cormos <nicolas@kdab.com>
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
BlackBerry: Added warning when low memory event is received

Low memory warning has been added for debug purposes.
Backport of 6b413dc32c485853710352a95e8bef928682558a

Change-Id: I79b3815d314227634b739935d85248cc526f2c89
Reviewed-by: Peter Hartmann <phartmann@blackberry.com>