Currently, when joining two armatures, the drivers of the armatures being merged
in are lost. This commit introduces a new AnimData API function for merging
animation data into another AnimData block.
NOTE:
* For now, this only copies the drivers over. As a result, manual effort will
still be needed to go through and fix the drivers.
I am working on automating that process, but it's more important that the
drivers don't have to be created from scratch for now (since this is needed
for the Goosberry rigging work).
3df93d063e