[LDCad] Rebrickable LDCad PBG export

[Merlijn Wissink]

- "-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

I don't know if all of it uses the API, but if you're running into throttling issues, you could always use (for parts of the application) the offline download files: https://rebrickable.com/downloads/

Only drawback is you need to keep them updated.