effects/desktopgrid: Replace highly opmitized animators with animations which work

Unfortunately, *Animator QML types have some problems with rendering,
sometimes affecting general smoothness, other times (like here, for me)
just not animating entirely. On my NVIDIA graphics system the animation
or a desktop returning from drag just looks like a freeze for a second,
then it suddenly jumps back to {0,0}. So let's just replace with less
optimal NumberAnimation which works fine.

FIXED-IN: 5.26
(cherry picked from commit 5cf43200)