Comment # 63 on bug 1030873 from Ludwig Nussel

In a discussion with Dominique we agreed for TW to

- get rid of DEFAULT_WM
- try to add some code to aaa_base to migrate the DEFAULT_WM setting to
update-alternatives. aaa_base already needs coreutils which is the only
dependency of update-alternatives anyways.
- remove the patch that reads DEFAULT_WM it from GDM
- make GDM fall back to default.desktop first, then potentially gnome.desktop
- apply the same logic to wayland sessions

SDDM already doesn't read DEFAULT_WM so broken in that regard already since
it's introduction