Label component and associated radio and checkbox actions

This introduces a simple ```Label``` component, a subclass of ```Text```, which adds only a ```target``` property and an event handler to call the target's ```receiveFocusFromLabel``` method when the label is clicked. 

This commit also contains implementations of responding to the label click for radio buttons and checkboxes. Obviously, many more components should be supported out of the box.

A label and its associated radio button can be configured like so:

```javascript
"name": {
    "prototype": "montage/ui/label.reel",
    "properties": {
        "element": {"#": "name"},
        "target": {"@": "selected"}
    }
},
"selected": {
    "prototype": "digit/ui/radio-button.reel",
    "properties": {
        "element": {"#": "selected"}
    }
}
```