Re-arrange the items in the Soundcloud panel into a more sensible order &
arrangement, and add a (not-yet functional) tick-box to set the uploaded
files to be downloadable.
Rename the Soundcloud-related members of ExportHandler to all begin with
soundcloud_, and rename a couple of members of SoundcloudExportSelector
too for consistency.
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
