Blender Git Loki

Python: add a utility function for creating custom properties.

Creating a fully functional custom property requires also setting up
its limits, description, default value and static override status.
It is complex enough to warrant providing an official utility.

Currently boolean properties are technically int, but the utility
pretends they are separate in case that eventually they are.

Reviewers: campbellbarton

Differential Revision: https://developer.blender.org/D4620