Restructure code to manually add staves, braces, and notes.

Previously, the program was hard-coded to draw a grand staff, (two staves
connected by a brace). Now, at the level of the score library, each staff
and any braces are added manually and any set of staves and braces can be
drawn, (the only the two-staves-connected-by-one-brace has been tested).

There's also functionality for ading notes, though no notes are drawn yet.