Refactor JSON serialisation

[Nick-Hall]

Simplify the JSON serialisation by creating methods to get and set the state of an object.

The method names get_attrs and set_attrs are not ideal and will probably need changing.

There is some copy and paste of these methods from the BaseObject into the Date object. Why are GrampsType, Date, StyledText and StyledTextTag not derived from BaseObject?

TODO: Look at removing the checks for empty dates from the to_json and from_json functions.

[dsblank]

Looking very clean!

[dsblank]

Why are GrampsType, Date, StyledText and StyledTextTag not derived from BaseObject?

I always wondered that too.

[DavidMStraub]

By the way, at the moment, the GrampsType values are still dependent on the locale, as we are not using xml_str. It would be nice if we could change that in the next db schema change, so the JSON becomes locale independent.

[Nick-Hall]

Yes. I've already changed GrampsType to store an integer value for pre-defined options and a string for custom values. This now mirrors the properties in the class.

[Nick-Hall]

I've just tried making BaseObject the parent class for GrampsType, Date, StyledText and StyledTextTag. The change to GrampsType causes errors so I'll probably leave it as it is for now.

[Nick-Hall]

Renamed get_attrs and set_attrs to get_object_state and set_object_state.