Make `musicxml2ly` support `<cancel>`

• define-grobs.scm: Fix distance between KeySignature and KeyCancellation

  MusicXML provides an option to position key cancellations after key signatures (for older French scores, to main the Circle of Fifths if the key changes from, say, five flats to two flats).

• musicxml2ly: Support <cancel> element of <key>

  This commit also sets the default to 'no key cancellation'.

  Also add a regression test (originally written by Michael Scott Cuthbert and taken from https://github.com/cuthbertLab/musicxmlTestSuite.git, with minor modifications).