Also add separate file to handle drop-logic.

#671

Mouuseenter, mouseleave for list and items are now always active and no 
longer added after the drag has started (they check of a drag operation 
is going on though). This makes the code simpler and more managable.

#634

#632

#631

#626

This fixes an issue where items could be added to only-one-item lists 
with replacement, when the  item to be replaced has the same list as 
origin.

#624