[kcmshell] Fix setting desktop entry

There's two cases we need to consider:

1) The KCM metadata is in <share>/kservices5/kcm_foo.desktop. In this case we set the full path to this file as desktop file name

2) The KCM metadata is in <share>/applications/kcm_foo.desktop. In this case we set 'kcm_foo' as desktop file name

This fixes showing the appropriate icon in the task manager for both cases