Move KItemRange to its own header

Move KItemRange to its own header

Also factor out the code that transforms a sorted list of ints to a
KItemRangeList. This removes some duplicated code from KFileItemModel.

Note that overriding operator<<() in KItemRangeList was necessary
because it's not a typedef for QList<KItemRange>, but a class derived
from that now, and some code fails to compile if the return type of
that function is QList<KItemRange> and not KItemRangeList.

REVIEW: 112728