Auto-construct Label widgets from strings

Just a little convenience for Lua. When building up a big widget structure (eg filling a `Box` or `Grid`), creates a `Label` for any strings it finds. So now you can replace:

````lua
box:PackEnd({ ui:Label("foo"), ui:Label("bar"), ui:Label("baz") })
```

with

```lua
box:PackEnd({ "foo", "bar", "baz" })
```