TIDOC-949 Added extends field and corrected subtype field.

Added a new "extends" field to show supertype.

Fixed the existing "subtype" field. It was supposed to show either "proxy", "view" or null, but the logic for view was incorrect. 

Testing notes:

1. Generate json file:

        sh docgen.py --format=json --output=<some_output_dir>

2. Check JSON file. Each type should have the correct supertype in "extends", and types that inherit, directly or indirectly, from Ti.UI.View should have subtype="view".