Added Schema Enforcement for Min/Max Occurrence and Choice Tags

For a project at my work, we needed the GUI editor to disable certain elements if a "maxOccurs" attribute was provided in the schema.  Likewise, we needed enforcement of the XSD "choice" tag so that users could not select multiple elements where only one was valid.  These changes have been implemented, along with pre-population of required elements and restricting deleting under the minimum allowed occurrence.