Added several keys to the new input system

This allows the player to use "down" keys for selecting ships in melee,
even though there is no "down" in battle.  Infrastructure for several
later enhancements is also in this patch.


git-svn-id: svn://svn.code.sf.net/p/sc2/code/trunk@1024 8092fc87-c524-0410-9efc-e669fe64eaf9

sc2/content/starcon.key

@@ -69,10 +69,12 @@ Lander-Right: key Keypad-6
 Lander-Weapon: key RightShift
 Lander-Escape: key Escape
 
-# Player 2's flight controls.
+# Player 2's flight controls.  We add the key 'd' to let him
+# move down in the super-melee ship selection.
 Player-2-Thrust: key e
 Player-2-Left: key s
 Player-2-Right: key f
+Player-2-Down: key d
 Player-2-Weapon: key q
 Player-2-Special: key a