New XML parsing machinery.
Use the name 'block_type' throughout consistently.
Only set XML attrs as object attrs if they are fields.
UsageStore abstract interface
Use the new name, construct_xblock
Use the new Plugin injection instead of @patch
Direct tests of parsing, and fix the parsing of content.
More docstrings.
XML serialization: outbound half.
Equivalent is a better concept here than equality.
Simplify the HTML in/out: now need an <html> tag.
With the new scenarios scheme, there's no need for empty workbench_scenarios methods.
Rename the UsageStore methods to make a little more sense.
Properly handle HTML content in the text content of the element.
Make usage_store and field_data required to construct Runtime.
construct_xblock et al, can now take an optional field_data to override the one stored on the runtime.
Go back to a classmethod for parsing.