Use copy() from the appropriate event subclass

This fixes a warning that pops up where the event argument is of type
EventMotion, etc. rather than of type Event.  gdk.Event.copy() warns
when passed an object of type other than Event:

  File ".../hamster/lib/graphics.py", line 2121, in  __on_mouse_move
    sprite_event = gdk.Event.copy(event)

Signed-off-by: Bryce Harrington <b.harrington@samsung.com>