[LDCad] Rebrickable LDCad PBG export

[Orion Pobursky]

The new alpha location is here:
https://www.ldraw.org/hidden-content/pbg-test.html

Note: The download button doesn't do anything (yet)

- "-1" is automatically appended if not found and a message generated
- The download button works now
- If a part isn't found, the messages will provide you a link to the part on Rebrickable.

Still to be done:
 - Polish up the UI
 - Look up the set name
 - Implement template replacement
 - Populate the cross ref file