new Units.Filter interface  for unit listeners to receive only useful events

this interface will be added to all UnitListeners so that they can filter units
events so that they can notified only of unit events from units of "interest"
for example a UnitAgent can filter to only receive events from its bindee unit
added Units.Filter getter to UnitListener (see previous commit)
units manager now filtering units events using listeners' filters
added missing filter to needs manager
added missing filter to scouting agent
added missing filter to supply building agent
added missing filter to Base agent