nixos/displayManager: deprecate separate options for default wm/dm

The upstream session files display managers use have no concept of sessions being composed from
desktop manager and window manager. To be able to set upstream session files as default
session, we need a single option. Having two different ways to set default session would be confusing,
though, so we decided to deprecate the old method.

We also created separate script for each session, just like we already had a separate desktop
file for each one, and started using displayManager.sessionPackages mechanism to make the
session handling more uniform.