EditSession : change `scrollLeft` / `scrollTop` and their events from `int` -> `num` to support sub-pixel precision

Some platforms, such as Chrome, are starting to support sub-pixel
precision and we may get non-integer values coming up from ace / the
dom.  We were actually calling `toInt()` to coerce these into integers
before but I think its best to let them propagate as numbers and let the
user decide what to do with them.