TODO

List of things that should be implemented at some point:
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 * Fix accidental announcement to not print accidentals if they were already
 stated in a previous measure. Currently, we print accidentals as they
 are present on the score according to the MusicXML file. This can actually
 be a feature, but for compliance with braille music, we should avoid
 reprinting them.
 * Detect repeated note groups in measures. See next item.
 * Support for note grouping according to the braille music standard.
 * Support for chord symbols (harmony).
 * A widget for rendering a Measure instance in visual notation on-screen (GTK).
 This should be integrated into the braille music editing GUI, so that
 changes in either format update the other representation.