We now use the following algorithm to derive the directory name of the
default merge dirs:

  - if the .menu file is "${XDG_MENU_PREFIX}applications.menu" or if
    the name cannot be derived (e.g. because the .menu file is called
    ".menu" only) we use "$XDG_CONFIG_DIRS/menus/"applications-merged"

  - in all other cases, we use "$XDG_CONFIG_DIRS/menus/XYZ-merged"
    (where the .menu file is called "XYZ.menu")