- more dartdocs for the drag typedefs

- more toStrings to aid debugging

- require the position for DragUpdateDetails since we were omitting it
  in some places

- add the primaryVelocity to DragEndDetails so that consumers don't
  have to themselves track the axis in question

- fix the velocity tracker so that it doesn't walk the null data.
  Previously, near time t=0 (which pretty much only matters in tests,
  but it does matter there) we would walk the velocity data and then
  also walk missing data, treating it as Point.zero with t=0.

- simplify some of the velocity tracker; e.g. instead of trying (and
  failing?) to clear the velocity tracker when the pointer stalls,
  just drop the data before a stall during the velocity estimation
  (where we redundantly had another bigger horizon anyway).