Generate encoder, decoder, and schema generator for object classes

[jpeiffer]

Similar to how JsonDynamicWidget automatically generates the code from the Widget properties, it should be possible for this package to generate the code for Theme based objects to simplify maintenance and make it simpler to handle upgrades for Flutter.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.