Blender Git Loki

Refactor BKE_blender into:

- BKE_blender_version.h (only version defines & versionstr)
- BKE_blender_copybuffer.h (currently only used for view3d copy/paste)
- BKE_blender_undo.h (global undo functions)
- BKE_blendfile.h (high level blend file read/write API)

Reviewers: sergey, mont29

Differential Revision: https://developer.blender.org/D1947