[#1378] Make PropertyMixin public.

Problem description
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We want PropertyMixin to be public so that we can reuse it for non-configuration stuff.

It was created for exporting the read/write configuration over JSON-RPC, but I think that we can reuse it for exporting other object over JSON-RPC using a CRUD interface.

Right now I need it for exporting server status over JSON-RPC.

This needs to be reviewed after #12 


Changes description
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I could not find a better name for PropertyMixin.
Also the current PropertyMixin is both a PropertyMixin (for leaf objects)
but also a SectionMixin (for non leaf objects).

A leaf object is an abject which has no other child properties.

Maybe this should be split in PropertyMixin and SectionMixin but i don't know if it is worth the trouble.


How to try and test the changes
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reviewers @alibotean 

Check that changes make sense.