track non-tool-based changes too. it's likely that these won't ever be undo-able, so i didn't fuss with EXTDocument.m for them, but we'll see.
move the homology-to-homology key-value generation routine into EXTMatrix, so it can be easily reused for mult. anno.s
upgrade the EXTPolynomialSSeq method -productWithLeft:right: to an EXTSpectralSequence method, giving a uniform way of requesting multiplication matrices
duplicates Bavarious's new differential drawing code to make multiplication annotations pick out dots corresponding to particular homology representatives. this now actually closes #75, for real.