Qt Designer: Fix shrinking row span of QGridLayout items

Previously, shrinking a row span by moving the bottom widget handle up
did not work due to the code checking the width of the item. Correct
to check on the height when creating the command.

Change-Id: Ieb1cf7eb0e78bb2edba7f2a7d7ff920f28ada509
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>