'Construct a new XModuleDescriptor. The only required arguments are the
system, used for interaction with external resources, and the
definition, which specifies all the data needed to edit and display the
problem (but none of the associated metadata that handles recordkeeping
around the problem).
This allows for maximal flexibility to add to the interface while
preserving backwards compatibility.
runtime: A DescriptorSystem for interacting with external resources
field_data: A dictionary-like object that maps field names to values
for those fields.
XModuleDescriptor.__init__ takes the same arguments as xblock.core:XBlock.__init__'

def __init__(self, *args, **kwargs):

    super(XModuleDescriptor, self).__init__(*args, **kwargs)
    self.previous_version = self.update_version = self.definition_locator = None
    self.xmodule_runtime = None
