This also makes `IDP_CopyProperty` the "opposite"
of `IDP_FreeProperty`, which is what I'd expect.
Two refactoring steps:
* rename IDP_FreeProperty to IDP_FreePropertyContent
* new IDP_FreeProperty function that actually frees the property
Reviewers: brecht
Differential Revision: https://developer.blender.org/D4872
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