Comment 3 for bug 799754

Sorry if I wasn't really clear, here is a better explanation:

It would be useful if lightdm default config (/etc/lightdm/lightdm.conf) could be overridden by default by a distributor shipped one (e.g. /etc/lightdm/lightdm-vendor.conf, or something like that).
If this file is present, then the main config file isn't used, and lightdm automatically uses the other one.

Then, it would give the following scenario:
* the default config is ok for Ubuntu, so probably nothing to do (or maybe register it with the lowest priority, e.g. "update-alternatives --install /etc/lightdm/lightdm-vendor.conf lightdm-vendor-config /etc/lightdm/lightdm.conf 20" in ubuntu-artwork.postinst)
* Xubuntu needs to change the default session, so it will ship a custom config file (e.g. /etc/xdg/xdg-xubuntu/lightdm/lightdm.conf), and register it as an alternative (priority greater than 20) to /etc/lightdm/lightdm-vendor.conf (e.g. "update-alternatives --install /etc/lightdm/lightdm-vendor.conf lightdm-vendor-config /etc/xdg/xdg-xubuntu/lightdm/lightdm.conf 50" in xubuntu-default-settings.postinst)
* Another derivative can do the same with its own priority, greater than 20, e.g. "update-alternatives --install /etc/lightdm/lightdm-vendor.conf lightdm-vendor-config /somewhere/other/derivative/lightdm.conf 60".

By default, the highest installed priority will win.

Thanks to that, an admin (or a user) with several *-desktop metapackages can choose its preferred vendor config by playing with "update-alternative --config lightdm-vendor-config", just like what can be done for $editor.

The changes are rather minimal in lightdm, I'll post a few patches soon (only init script changes).