Convert the new filename to native separators before checking it

If a native separator was put in the new name when renaming a file name
via the file dialog then it would correctly fail. But if a non-native
one was used on Windows then it would cause the file to be moved
instead if the directory existed.

Change-Id: If01760b8c54a69b600c9a44c7509017be70d33e3
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>