Toggle roiEnableButton and zoomControl when coordinates updated

Resolves the issue reported here: https://groups.google.com/d/msg/igv-help/eqwVT7_Y1d4/rMvY9CVN8x8J, where roi button was persistently disabled.
Removed "Affective" classes.
Remove superfluous Genome interface,  there is only 1 implementation
Support defining minimal genomes from chrom.sizes files