Implement style method for default text block styles

This pull request addresses #303. It sets default styles on text block classes, so that elements of that class use these defaults (though you can override them).

See samples/sample56.rb for a test

In my testing, Shoes 3 does not allow you to use this method to set defaults for shapes or backgrounds. These defaults do not apply, for example:

```ruby
Shoes.app do
  style Background :margin => 20, :fill => orchid
  background lawngreen
end
```

So, for now, anyway, this method only has effect on TextBlock elements.