Poptip dragresize

Retarget of https://github.com/palantir/plottable/pull/863 from internal branch so we can all work on it easily
resizeXEnabled is whether it is possible for the dragBox to be resized. xResizing is whether the dragBox is presently resizing. if dragBox is resizing, then xResizing is an object which has some info on the resize that is occuring. so we coerce to boolean to satisfy the boolean contract