Improve performance while switching item arrays

Found some bits of performance while searching for the item vertical swapping issue.
With that changes the move/float will only be called once per sizeChanged watch giving about 15% - 20%
less while loading the main sample. On bigger grids, it will be even more.

In addition to that, the setSizeX/Y where called on positionChanged. Removing them did not change the behavior of the samples.
Any reason that these calls were in positionChanged?