Leave the down_ event set

The other "up" type listeners (for touch and pointer) do not set this to null.  This event can be of use to a click listener that might want to compare the down position to the click event's position.
Provide a method to get the most recent down type event

This will allow interactions to conditionally handle later events based on the down event.
Merge pull request #1252 from tschaub/expose-down

Add a getter for the most recent down event.