Instead of letting each qmake variable have its own auto-generated name
we try to group common variables into similar groups as used by the Xcode
templates provided by Apple.
We also prevent the same files from ending up multiple times in a group.
Change-Id: I73b13d6071bb7b3cd1501c422a99c60743221485
Reviewed-by: Andy Shaw <andy.shaw@digia.com>