KitManager: Introduce sortedKits() method

This allows to delay the evaluation of displayNames (which might be
expensive now that we expand them) to when a sorted list is actually
needed, thus speeding up the start-up of Qt Creator.

Change-Id: I78273592bc0f8311372f8d799faf9e1289759013
Reviewed-by: hjk <hjk121@nokiamail.com>