Blender Git Loki

Fix for [#26712] Particle group instance 'Use Count' value gets reset on file-load.
* New object pointers can't be loaded properly for library linked groups, so the weight groups now store an index to the group objects at save time. This index is used at load time to set the objects without relying on the old pointers.
* If the library linked group is modified the indices can be wrong, but this can't really be avoided easily as there's no way to relate objects in a linked group between loads.