Make MusicXML the master format

[sertansenturk]

• Make sure MsuicXML has the complete information from other formats, e.g. embellishments, legatos indicating the continuous singing of a phrase. Note: not all of them are in the txt format. However it might be easier to add them through a note editor by hand.
• Make sure the scores are visually non-problematic, e.g. scores have beaming info, natural/accidentals are displayed correctly, the order of the key signature is maintained, the lines should break in the subdivision of long usuls. The repetitions should also be written once when appealing.
• Include scripts to generate other formats, e.g. txt, mu2, pdf, midi, abc. Remove all formats except MusicXML from the repository. These formats should be generated separately and distributed only for upcoming releases.

[sertansenturk]

We should explicitly write how much of these will be fulfilled by @burakuyar's MusicXML converter.