justDown property indicates if a key has been newly pressed since last time you checked

Added justDown access to _justDown internal boolean.  Records whether the key has been newly pressed since the last time it was checked.  Resets to false only when it is read.  Used for anything that needs to know when a key is first pressed with no autorepeat, without using closures or callbacks.
Possible alternative names for the property would be: newDown, or newPress
This makes `justDown` incorrect if called multiple times. The other `just*` functions check the time the event occurred. In this case that would be utilizing `Key.timeDown` and `Key.duration`.