nixos/displayManager: introduce defaultSession

There's two ways of providing graphical sessions now:
- `displayManager.session` via. `desktopManager.session` and
  `windowManager.session`
- `displayManager.sessionPackages`

`sessionPackages` doesn't make a distinction between desktop and window
managers. This makes selecting a session provided by a package using
`desktopManager.default` nonsensical.

We therefor introduce `displayManager.defaultSession` which can select a session
from either `displayManager.session` or `displayManager.sessionPackages`.

It will default to `desktopManager.default + windowManager.default` as before.
If the dm default is "none" it will select the first provided session from
`sessionPackages`.