Blender Git Loki

WIP: Refactor IDProperty UI data storage

The way IDProperty UI data (min, max, default value, etc) is quite
complicated. For every property, retrieving a single one of these values
involves three string lookups. First for the "_RNA_UI" group property,
then for another group with the property's name, then for the "min" or
other value name. Not only is this inefficient, but it's very hard to
figure out, unintuitive, and just generally not helpful.

This commit replaces that system with a UI data struct directly in the
IDProperty. If it's not used, the only cost is of a pointer. Beyond storing
the description, name, and RNA subtype, derived structs are used to store
type specific UI data.

This patch currently makes more changes than I would make in the first
step of landing this refactoring to master, and it doesn't consider
versioning yet.

Differential Revision: https://developer.blender.org/D9697