lightdm updates need to review.

[Feng Shu <tumashu@163.com>]

Hello everyone.

I have try many display-manager in guix: gdm, sddm, slim and lightdm,
and finally find lightdm is very comfortable to use after it fix session
menu dispear problem in gux:

1. it start very fast.
2. lightdm-gtk-greeter can change font, background easy.
3. it can support other greeters.


I have send the below patches, please review them if someone have time,
thanks,

1. Add lightdm-greeter-general-configuration and simplify code

    https://issues.guix.gnu.org/75048


with this configuration, user can use other greeter, for example:
slick-greeter, lightdm-mini-greeter, lightdm-tiny-greeter or
lightdm-pantheon-greeter easily.

2. Add slick-greeter

    https://issues.guix.gnu.org/75043

3. Add lightdm-tiny-greeter

   https://issues.guix.gnu.org/75417

4. Add lightdm-pantheon-greeter 

   https://issues.guix.gnu.org/75186


For I use xfce4, so I just package some greeter using gtk, and do not
package greeter using qt, if someone interesting, can try :-)

the below is lightdm configuration to test:


```
(service lightdm-service-type
               (lightdm-configuration
                (greeters
                 (list (lightdm-greeter-general-configuration
                        (greeter-package slick-greeter)
                        (greeter-session-name "slick-greeter")
                        (greeter-config-name "slick-greeter.conf")
                        (config (list "[Greeter]"
                                      "font-name = San 12"
                                      "background = /run/current-system/profile/share/backgrounds/guix/guix-checkered-16-9.svg")))
                       (lightdm-greeter-general-configuration
                        (greeter-package lightdm-mini-greeter)
                        (greeter-session-name "lightdm-mini-greeter")
                        (greeter-config-name "lightdm-mini-greeter.conf")
                        (config (list "[greeter]"
                                      "user = guest"
                                      "[greeter-hotkeys]"
                                      "mod-key = control"
                                      "session-key = e")))
                       (lightdm-greeter-general-configuration
                        (greeter-package (customize-lightdm-tiny-greeter #:session "icewm"))
                        (greeter-session-name "lightdm-tiny-greeter")
                        (greeter-config-name "lightdm-tiny-greeter.conf")
                        (config (list "## Lightdm-mini-greeter have no config, ignore it!")))
                       (lightdm-greeter-general-configuration)
                       (lightdm-gtk-greeter-configuration
                        (lightdm-gtk-greeter lightdm-gtk-greeter-gee)
                        (extra-config
                         (list "font-name = San 10"
                               "icon-size = 64"
                               "xft-dpi = 140"
                               "clock-format = %Y-%m-%d %H:%M"
                               ;; We need to use "~~" to generate a tilde, for
                               ;; extra-config sting will be handle as
                               ;; control-string of format function.
                               "indicators = ~~host;~~spacer;~~session;~~a11y;~~clock;~~power")))))
                (seats
                 (list (lightdm-seat-configuration
                        (name "*")
                        (greeter-session 'lightdm-tiny-greeter))))

                (xorg-configuration
                 (xorg-configuration
                  ;; The QXL virtual GPU driver is added to provide a better
                  ;; SPICE experience.
                  (modules (cons xf86-video-qxl
                                 %default-xorg-modules))
                  (keyboard-layout keyboard-layout)
                  (server-arguments
                   (append %default-xorg-server-arguments
                           '("-dpi" "140")))))))


```


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