MusicXML: Fix problems with chord detection and pickup measures

MusicXML: Fix problems with chord detection and pickup measures

Pickup measures messed up chord detection and voice start positions
for multi-voice parts, because I erroneously reset the position after
a pickup measure to 1, while in fact it should have been the length
of the pickup measure!
Also, fix some typos and only ignore skips in pickup measures intended
for padding to a full measure. Explicitly adde skips need to be honored.