Introduce getAttributeTypeDisplayName

Let's add the getAttributeTypeDisplayName method to the AttributeType
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 getAttributeTypeDisplayName
Merge pull request #1445 from mlocati/add-AttributeTypeDisplayName

Introduce getAttributeTypeDisplayName