QtIFW

Fix wizard's maximum size

User can set height and width for wizard in config.xml. If the given
dimentions does not fit to the screen, wizard was clipped and buttons
were of the screen. Fixed so that the screen size cannot be exceeded.

Task-number: QTIFW-1016
Change-Id: Iab6970d4203fc1271b93475aec71c946d0508653
Reviewed-by: Iikka Eklund <iikka.eklund@qt.io>