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).
Remove the json encoding option from getAttributeKeyDisplayName
Merge pull request #1443 from mlocati/add-AttributeKeyDisplayName

Introduce getAttributeKeyDisplayName