KFileItemModel: Always use upper case letters to group by name

KFileItemModel: Always use upper case letters to group by name

This fixes the following problems:
1. In a folder containing the items ~a and b, the first group,
   containing ~a, is named "Others".
2. In a folder containing the items a, ~b, ~c, and ~d, the first group
   "A" contains a and ~b, and the other two items are in the group
   "Others".