nixos/gdm: make desktopManager.default work

Unfortunately, you can't configure the default user-session
with GDM like lightdm. I've opened a feature request [0]
but I'd like to be able to do this now.

We use a GObject Python script using bindings to AccountsService
to achieve this. I'm hoping the reliable heuristic for session names
is the file's basename. We also have some special logic for which
method to use to set the default session. It seems set_x_session is
deprecated, and thusly the XSession key, but if that method isn't used
when it's an xsession it won't be the default in GDM.

[0]: https://gitlab.gnome.org/GNOME/gdm/issues/535