The desktop file name should not contain ".desktop" suffix, but some
applications still specify it anyway because of the ambiguity in the
documentation that was fixed in
0c5135a9df.
This change makes setDesktopFileName remove ".desktop" suffix so
desktopFileName always returns a desktop file name with correct format
and its users don't need to chop ".desktop".
Pick-to: 6.5
Change-Id: If5abccaf3bf976449cada8891fff887870e45b5f
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
fb86c5a470