[bug#35305] LightDM service

[L p R n d n]

Hello,

Here is the new set of patches with quite a few modifications for the
service, everything rebased on 1.1.0.
After a discussion with bricewge on irc, a new design has been adopted
allowing use of multiple seats.
To use the service, you can instantiate a lightdm-service-type, a
lightdm-gtk-greeter-service-type or both. The service should always find
a way to start a working instance of LightDM. :) 
A non exhaustive list of changes that might need further review:

* lightdm-gtk-greeter has its own service extending lightdm-service and
  others.
  
* Seats configuration use now the lightdm-seat-record-type and can be
  defined both in the lightdm-service and lightdm-gtk-greeter-service.
  The greeter now extends the LightDM's service by providing a list of
  seats.

* The `compose` and `extend` field of the lightdm-service-type have seen
  work for this purpose while keeping compatibility with
  `set-xorg-configuration`.

* extra-config fields are list of strings to avoid weird indentation in
  the user's config.

* Fixed customization of lightdm-gtk-greeter's cursor.

* Added some fields to deal with accessibility for
  lightdm-gtk-greeter-configuration.

* Takes default background from guix-artwork.

Also a review of PAM services might be nice as I'm not versed in security.

However, I didn't get rid of etc-service-type's extension as LightDM's
seems to ignore `sessions-directory` when configuration is passed
through `--config`. Patches to fix that are welcome but can wait a
future patch submission, I think.

Otherwise, tested in a WM: default configurations, autologin, sway,
changing greeter appearance and everything worked fine. For passwordless
login, had to actually delete user's password manually to make it work.
I don't know if it's expected.

Have a nice day and good review :)


L  p R n  d n

0001-gnu-lightdm-Update-1.30.0.patch
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0002-gnu-lightdm-Add-vala-bindings.patch
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0003-gnu-lightdm-Disable-python-tests-only.patch
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0004-gnu-lightdm-gtk-greeter-Fix-at-spi-runtime-dependenc.patch
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0005-gnu-lightdm-gtk-greeter-Fix-.desktop-file.patch
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0006-gnu-lightdm-gtk-greeter-Wrap-binary.patch
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0007-gnu-lightdm-Build-accountsservice-files.patch
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0008-gnu-lightdm-gtk-greeter-Fix-some-warnings.patch
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0009-services-Add-lightdm-service-type.patch
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0010-gnu-lightdm-gtk-greeter-Set-XCURSOR_PATH-in-wrapper.patch
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