Data model improvements, copy items with location move.

Do away with the bidirectional mapLocation<->item relation. Only
mapLocation->item is required.

When a map location is dragged, bring its items with it.
Delete location item. Add character data.

Delete location item by dragging from the location properties window to
the wastebasket.
Add character data.
drag & drop enhancements

Ability to drag items to a new location.
Do away with the bi-directional link between item & location.
Code tidyup and refactoring.
Refactoring