Fix importing and rendering cross-system beams

[dmitrio95]

This pull request contains patches necessary to support cross-system beams, which fixes #388 and other similar crashes. With these patches the relevant systems from the score referenced in #388 looks like this (note the beam between the last note of the first system and the first note of the second system):

Apart from the changes that implement rendering cross-system beams, this pull request also improves importing cross-measure beams from MusicXML. Presence of cross-measure beams may lead to having multiple beams simultaneously open when parsing a MusicXML document, which can lead, without these changes, to some beams being mixed up:

[cecilios]

Thank you. The code looks right to me, and all regression test are passed. I will merge.

[dmitrio95]

Thanks for merging!