Improve quality of beams within the stave by using staff line positions

[Silverwolf90]

Beams look pretty poor. Our current algorithm for calculating the slope is completely independent of the staff. However, best practice says that the start and end of a beam are placed on the lines.

See Behind Bars, page 17, Beam Placing within the Stave,

A horizontal beam should not be placed in the middle of a space. The closeness
of beams and stave-lines is uncomfortable to read [...] A beam must be attached to the top or bottom of a stave-line or else be centred on the line.