care for new sound instances

without creating new ones, we’d need instead to care for rewinding,
which could get quite complex.
try to fix number of rows for middle cols

not sure what's the right number, but it’s probably not 17
small typos and adjustments

really just details
remove square delay from instance

just to re-enforce it isn’t needed
floatPos is needed just to identify end position

so better explicitly remove it from start, to let it clear we don’t
need it there
detail when drawing the square

float position is good for debugging
3 more instance squares

they will look like just one
remove null bug

remember to always check for nulls, when relevant
separate into files

better organized this way
few better file comments

for Grid and SongScript, at least
remove possible Sound bug

trying to construct an empty sound should yield to a compile error,
instead of this silent non-warning