[REG 3.3 -> 3.4] Renaming file doesn't work correctly when changing case only

1. Create a new Qt Widgets Application project.
2. In Projects view, right-click on mainwindow.h and select "Rename..." from the context menu.
3. Type "MAINWINDOW.H" and press Enter.
   The file will be renamed correctly in the file system and in the pro-file. In Projects view, it doesn't change, though.
4. In Projects view, right-click on mainwindow.h and select "Rename..." from the context menu again.
5. Type "somefile.h" and press Enter.
   The file will disappear from the project.

Renaming a file should still work correctly, even if only the case of the letters changes.

Found by Squish test tst_rename_file