Blender Git Loki

UI: Support drag & drop reordering of collections

This adds initial support for reordering collections from the Outliner
using drag & drop.
Although drag & drop support is limited to collections for now, this
lays most foundations for general drag & drop reordering support in the
Outliner. There are some design questions to be answered though:
* Would reordering of other data types (like objects) be a purely visual change or would it affect the order in which they are stored? (Would that make a difference for the user?)
* Should/can we allow mixing of different data types? (e.g. mixing render layers with objects)
* How could we realize this technically?

Notes:
* "Sort Alphabetically" has to be disabled to use this ("View" menu).
* Reordering only works with collections on the same hierarchy level.
* Added some visual feedback that should work quite well, it's by far not a final design though: {F493806}
* Modified collection orders are stored in .blends.
* Reordering can be undone.
* Did minor cleanups here and there.