added valueOf(String keyname) and toString(int keycode) to Input.Keys

Allows getting a string representation of the hardcoded int keycodes without using an enum.

one concern: BUTTON_CIRCLE and F12 both map to 255, I chose to handle it by ignoring BUTTON_CIRCLE, since F12 is much more likely to be pressed (what is BUTTON_CIRCLE anyways?)

~~The lookup in valueOf could be made faster (it iterates through all keycodes), but using an IntMap means minimal memory footprint, and it should be fast enough for vast majority of cases.~~