Upon close inspection, the way the Offset mode works in the
Copy Rotation constraint makes no sense, and in fact, destroys
the rotation of its owner unless either it's single axis, or
the order is set specifically to `ZYX Euler`.
Since it can't simply be changed because of backward compatibility
concerns, replace the checkbox with a dropdown that provides a set
of new modes that actually make sense.
Specifically, add a mode that simply adds Euler components together,
and two options that use matrix multiplication in different order.
The Python use_offset property is replaced with compatibility stubs.
Reviewers: brecht
Differential Revision: https://developer.blender.org/D5640
Part of T68836
`transform conversions.c` is a file that is getting too big (almost 10,000 lines).
So it's a good idea to split it into smaller files.
differential revision: https://developer.blender.org/D5677
