Introduce getPermissionKeyDisplayName-Description

Let's add the `getPermissionKeyDisplayName` and `getPermissionKeyDisplayDescription` methods to the `PermissionKey` class.

They return the localized name/description (so that you don't have to call `tc()`) and they optionally escape the name/description 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-.../