Proper fix, using `RNA_def_property_enum_bitflag_sdna` to mask the
affected bitflags when setting the value, and not overwrite unrelated
other bitflags.
Also similar fix/cleanup in DataTransfer modifier, who was also using
custom setters to handle this masking, instead of defining the
properties with `RNA_def_property_enum_bitflag_sdna`.
And added some documentation to `RNA_def_property_enum_sdna` and
`RNA_def_property_enum_bitflag_sdna`, to try to make it more explicit
what they are doing and what are their expected usecases.
474abdd7eb