Blender Git Loki

Unified code for identifying unconventional shapekey owners between
gooseberry and alembic branches.

The shapekey code has been modified in both branches, adding particle
shapekeys in gooseberry and cache library shapekeys in alembic. The
two approaches are slightly different, but based on the same idea of
extending the 'from' owner info in Key, so that keys can be used without
a 'from' ID and/or with an index for identification.

This patch unifies the approach in both branches to make it slightly
less messy and avoid merge conflicts.