Sortable: Calculating item distance and direction using a more robust algorithm to...

Sortable: Calculating item distance and direction using a more robust algorithm to better support sorting among nested sortables. Fixes #8572 - Wrong placeholder positions. Fixes #8573 - Can't drag an item out of an inner sortable. Fixes #8574 - Hard to put an item between two inner sortables.

Use the item which has the least distance between the mouse
pointer and one of its borders to rearrange, with direction being
determined by the nearest border.
Also we use this algorithm to rearrange even when currentContainer
is not changed to override the defective rearrangment in
_mouseDrag