Working Soundcloud export

Working Soundcloud export

Adds an 'upload' property to ExportFormatSpecification, to indicate that files
exported with that format specfication should be uploaded to Soundcloud, and
makes it editable in the export format dialogue.
Adds fields for the Soundcloud username & password to the file format selection
page, as well as an option to make the uploaded files public and open them in
the system browser.

Possible improvements not yet implemented:
- make upload happen in its own thread
- cosmetic tidying up of dialogue control layout
- remember username & password