First, instead of modifying all the get_constraint_targets and flush_constraint_targets callbacks, introduce wrapper functions for accessing constraint targets, convert all code to use them, and handle the new reference there uniformly for all constraints.

This sounds like a nice non-functional change.

This incidentally revealed a place in the Collada exporter that didn't clean up after retrieving the targets and fixed it.

Good find :) Fixing this should be done in a different patch, though.

Also, tag the special target with a flag so other code can handle it appropriately where necessary. This for instance allows dependency graph to know that the Use B-Bone Shape option doesn't affect this specific target.

This seems to be a functional change, and thus should be done in a separate patch.

Finally, rename and simplify the function for initializing the custom space, and make sure it is called everywhere necessary.

This is yet another change, and should thus be done in yet another patch.