#1398 make variants movable (state: open)

make variants movable: currently, variants (children) cannot be moved to another variant or a child cannot be made a parent

Making variants a base should not be a big deal, moving variants between base items will be harder I guess, as we will then have to manipulate not only the varbase field but also all varianting attribute values in the paste handling.

we also have the problem that we only have "manual sorting" to move - you don't really want that especially since you also have to prevent child elements from becoming child elements... you would need a special "moving icon"