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Outliner: Modifier/constraint/shaderfx drag and drop operator

This adds an operator to allow drag and drop of modifiers, constraints,
and shader effects within the outliner. Referred to as "data stack" in
the code for simplicity.

The following operations are allowed:
* Reordering within an object or bone
* Copying a single modifier/constraint/effect to another object or bone
* Copying (linking) all modifiers/constraints/effects to another object
or bone.

This complements the recent work done for panel-based modifier layouts
by allowing reordering in the outliner. It also makes it simple to copy
a single modifier/constraint/effect to another object.

Differential Revision: https://developer.blender.org/D8642