Support tie and slur

[oxygen-dioxide]

MusicXml is built for sheet music visualization instead of playing. When playing or singing sheet music, tied notes should be treated as one note, and slur notes should be treated as one syllable. However, scores parsed by MusicXml.Net know nothing about ties and slurs, making it unable to correctly play the music.

[vdaron]

I'm working on notations that will gives you these information