[GP] Time signatures with compound-time are corrupted in .gpx format

Nightly 421f366 / Windows 7

1) Load this file (created from scratch with GP6): compound-time.gpx

Result: all time signatures with compound-time (eg 9/8, 12/8) and other "customized" time signatures hence the numerator is = or > to 7 ("seems": I have not verified all in detail) are corrupted.

- Works with 6/8 (see measures 10 and 11 in the test file) and with other commons time signatures.