QueryUi: Drag and Drop: Filter item retains filter after getting dragged

[rpbouman]

Repo:

1. create query with a filter
2. drag filter item to an axis
3. remove original filter item
4. drag axis item dragged in 2 from the axis back to the filters axis.

Notice that in 4, the original filter is restored.
One could think of it as a feature, but I think it is too obscure. Filters should probably be removed when dropping the item on the axis.