[kio_file] Handle renaming file 'A' to 'a' on FAT32 filesystems

Summary:
Use QFile::rename() to handle so renaming a dir/file 'A' to 'a' on
case-insensitive filesystems, e.g. FAT32/VFAT.

This code used to be in CopyJob but now has found a new home in kio_file,
see the diff review for more details.

Test Plan:

• jobtest unit test passes again
• Renaming a dir 'A' to 'a' on a FAT32 partition still works

Reviewers: Frameworks, dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D29610