Key signature support in older versions of Guitar Pro

Flats occurring in key signatures in Guitar Pro were handled incorrectly, which led to bug [#23519](http://musescore.org/en/node/23519). This fixes that, and tests are included.

Also allowed the GPX extension to be supported by the test framework, not committing a test for GPX at this time as there as I still have warnings printed out to indicate when something has been only partially supported.