Let the Category generate its own push data structure.

The Category Option and Value domain objects now have means enabled
for creating their own datastructures which will be converted to
proper JSON when creating a new item or updating an existing one.
Generalized the way changed data is collected for pushing to server.

Basically moved the responsibility to each item and field to collect the changed
values from its children. For this the "Pushable" interface has been introduced.
The Category field has also successfully implemented the save flow.
Removed debug log prints.