allow two KeyBindings for each KeyAction
update string <-> key binding conversion to cope with two bindings
update InitKeyBinding to allow two default bindings for key actions
expose both key action binding slots to Lua
make KeyBinding::Description return an empty string for a disabled binding
make the BindingCapture widget's attributes nil if no binding is set
extend Engine.SetKeyBinding to accept two key bindings
update the control options screen to support two key bindings per action