Bug 1022609 - Add multiIME field for keys

This way, then you could not define a key that has a keycode for multiIME, because it would accept only a multiME that contains a char.
For example, what if we would need to define this in layout,
```js
// some special symbol
 {
    value: '+',
    multiIME: {     // alternative for multIME
            // changed to some special, say [Backspace]
            value: '⌫', ratio: 1.5, keyCode: KeyEvent.DOM_VK_BACK_SPACE
     } 
 } 
```

This is just an example, but hope you get my point here.