deeper zoom
more sensitive zoom in FF
reduce zoom drift when pointer is over window
make horizontal frame padding smaller
more robust TimelineFrame.getChildRange()
simple performance.now polyfill for demo

(put that somewhere more global? utilities?)
more robust getJSON for demo
timestamp passed to enter/leave defaults to perf.now if 0. this might mess up profile imports from browsers

might check for undefined in enter/leave, but this affects only data imported from profiles so probably better to hack it there.
implements kind visibility (filter)