Rename OSLeft/Right to MetaLeft/Right

[garykac]

Since we renamed 'OS' to 'Meta' in for the |key| values, it makes sense to do the same for the |code| values.

Making this change would make all the modifier keys consistent between |key|, |code| and the modifier flags.

[garykac]

To clarify, we currently have:

key = Control; code = ControlLeft, ControlRight
key = Shift; code = ShiftLeft, ShiftRight
key = Alt; code = AltLeft, AltRight
key = Meta; code = OSLeft, OSRight

And one of these things is not like the others: Meta and OSLeft/Right is clearly an oddball.

With this change, we would have:

key = Control; code = ControlLeft, ControlRight
key = Shift; code = ShiftLeft, ShiftRight
key = Alt; code = AltLeft, AltRight
key = Meta; code = MetaLeft, MetaRight