Don't update size when minimum size changes while animation is running

Changing the width/height while animation on width/height is running
will cancel it. This fixes bug with applet being moved on top of another
applet when move operation results in animation that changes the applet's
position and size.

+ small cleanup: width/height animation "to" is always 0

no need to twice calculate grid position for positionItem()

Differential Revision: https://phabricator.kde.org/D989