Introduce getAttributeKeyDisplayName

Let's add the `getAttributeKeyDisplayName` method to the `AttributeKey` class.

It returns the localized name (so that you don't have to call `tc()`) and it optionally escapes the name in html format, javascript format (default is to escape in html format).

Discussion: http://www.concrete5.org/developers/beta/beta_discussion/retrieving-display-names-of-pages-attributes-.../