Introduce getAttributeSetDisplayName

Let's add the getAttributeSetDisplayName method to the AttributeSetKey
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).
Don't export localized attribute set name

Once the value is exported and then imported, it will be translated when
you print the set name.
Remove the json encoding option from getAttributeSetDisplayName
Merge pull request #1444 from mlocati/add-AttributeSetDisplayName

Introduce getAttributeSetDisplayName