Show Soundcloud credentials and upload options only when required

Show Soundcloud credentials and upload options only when required

A slightly hacky patch to make the Soundcloud username & password and the
upload options only become visible when at least one export format is
chosen for upload to Soundcloud.

I think that changing ExportDialog::soundcloud_selector to a
boost::shared_ptr is the right thing to do, but I must confess to having
only a hazy understanding of what boost::scoped_ptr was doing in the first
place.

Also it feels a bit wrong to be passing a pointer to ExportDialog's
SoundcloudExportSelector around, but I can't (for the moment, at least)
think of any better way to affect its visibility from inside
ExportFileNotebook.