Blender Git Loki

Port particle instance modifier changes from Gooseberry branch

The work is mainly from Lukas Toenne, with some modifications from myself.

Includes following obvious changes:

- Particle system selection is now name-based, with lookup menu.
- Lots of new options to control varieties.

Changes comparing to the Gooseberry branch:

- Default values and versioning code ensures same behavior as the
old modifier.

- Custom data layers are coming from vertex color, the modifier
does not create arbitrary layers now. The hope is to keep data
more manageable, and maybe make it easier to select in the shader
later on.

This means, values are quantized to 256 values, but it should be
enough to get varieties in practice.

Reviewers: brecht, campbellbarton

Reviewed By: brecht

Subscribers: eyecandy

Differential Revision: https://developer.blender.org/D3157