all times in KeyboardDragListener should be in milliseconds

[zepumph]

Discovered while working on phetsims/gravity-force-lab#186. There are some values in step that are in seconds, and some in milliseconds. Today I talked to @jessegreenberg about it, and we thought about converting it to seconds because that is what the sim gives us in the step function. After poking around I don't think that is the direction we should go. All the following are public options, and seem nice as millisecond values:

• hotkeyHoldInterval
  *moveOnHoldInterval
• moveOnHoldDelay

And each of these have an internal counter that is stepped. I think that these should continue to be in milliseconds.

In which case, we should see if the whole file can run in ms.

[zepumph]

This small change seems good for the most part. For the velocity, @jessegreenberg and I thought it best not to change the public facing apis from "pixels/second" (i.e. dragVelocity), so I ended up just converting within the step function and it is sorta a wash. I don't think it is too horrible though, and this way you have 5 sections in a row doing actions from ms instead of 4 from ms and one from dt. @jessegreenberg please review. Close if all looks good to you.

[jessegreenberg]

Change looks good and has been working for a long time. Other scenery/PhET APIs like FireListener and axon timers are in ms so this was a good change. At the same time, the "velocity" options in pixels per second is also most intuitive.

Closing.