improve the way blank pages are declared

[g-raffy]

At the moment, blanck pages in scan pdfs are declared as if they were a fake track (with a disabled keyword). As track ids are dictionary keys, they can't have the same name. As a result, we have to use a meaningless name to ensure there's no duplicate keys.

As an example, I had a scan of eb clarinet which had 2 blank pages : one at the beginning of the eb clarinet, and one at its end. Because blank pages are treated like disabled tracks, I've named them 'eb clarinet 10 and eb clarinet 11', as they need to also obey track naming conventions. This is suboptimal to say the least...