Blender Git Loki

Fix T91054: Editing group custom property gives error

This commit fixes the custom property edit operator for the the case of
editing group properties. Currently this isn't supported very well, the
data is converted to a string, but the operator shouldn't fail anyway.

This allows editing properties created like this:
C.object['abuse'] = {'parent' : ['child1', 'child2']}

These changes reflect some issues with the design of the operator.
Requiring guessing the type of the data does not work well at all, and
makes code more complicated. In the future this operator can be updated
to use a type drop-down.

Differential Revision: https://developer.blender.org/D12364