New module proposal: LightDM

[Robert Ancell <robert ancell gmail com>]

I'm proposing LightDM [1] as a replacement for GDM.  I started the
proposal for this in GNOME 3.0 [2] but due to the young age of the
project I thought it better to wait until 3.2 before making a full
proposal.  This is it.  I apologise this has been done after the
proposal period.

Why replace GDM?

- LightDM is a cross-platform solution.  Ubuntu is planning to switch
to it this cycle, and other distributions have expressed interest in
the project.  By sharing this piece of infrastructure GNOME can spend
more time working on important GNOME components.  LightDM is aligned
with freedesktop.org.

- I am confident that the LightDM architecture is simpler than GDM.
Some indicators of this:
  - Smaller code size
  - Well defined interface between greeter and session
  - Less dependencies
  - Less internal interfaces
  Architecture can be a personal opinion, and I encourage those with
programming experience to look at the code and decide for themselves.
Note that LightDM is not lighter in features, but in architecture.

- By having a well defined interface between the greeter and daemon,
it is significantly easier to develop a greeter without knowledge of
how display management works.  This is useful as the skillset and
motivations of these two sets of developers are different.

- LightDM is a platform for future work and is investigating the use
of new technologies like Wayland.

The details:
Purpose:  Cross-desktop display manager
Target: desktop
Dependencies: libglib, libpam, libxdmcp, libxcb, libxklavier,
gobject-introspection, libgtk+
Resource Usage: Launchpad for source control and bug tracking [1],
tarballs in public ftp [3] (in process of moving to freedesktop.org)
Adoption: Accepted for use in Ubuntu 11.10, interest from other distributions
GNOME-ness: Display manager is cross-desktop, example GTK+ greeter is
fully GNOME compliant.  I would recommend this module is maintained in
the GNOME servers to get all the build and translation support.
3.0 readiness: GTK greeter currently using GTK2, but all other code
uses latest GNOME standards.
License: GPL3

[1] https://launchpad.net/lightdm
[2] https://mail.gnome.org/archives/desktop-devel-list/2010-October/msg00226.html
[3] http://people.ubuntu.com/~robert-ancell/lightdm/releases/