fix GranularRectLayout getByCoord not taking into account rows that are allFilled
fix CanvasFeature mouseover events in firefox.  fixes #311
make File->Open GFF3 files default to using CanvasFeatures
fix JS gff3 parsing strand
when the track-pinning underlay is active, add a second gridlines track that draws gridlines and the global highlight on the pinning underlay.  fixes #310