Custom properties on pose markers in Actions were not handled properly
in the following ways:
- They were not written to or read from blend files, which resulted in
crashes (reported in #138201).
- They were not duplicated when the pose marker lists were duplicated
(during Action duplication), which would leave the duplicate marker
*sharing* custom properties with the marker it was duplicated from.
This PR fixes these issues by creating functions to handle the reading,
writing, and copying of marker lists which properly read/write/copy
custom properties as well, and using those functions in the relevant
places.
Pull Request: https://projects.blender.org/blender/blender/pulls/138494
4e596d18c1