main/postmarketos-ui-{mate|xfce4}: switch to slick-greeter

This is a long-standing change resulting from #1964.

Actually the goal is to ease enabling/disabling autologin. With "lightdm-settings" installed, there is an administration/settings GUI "Login Window" available that allows among others to configure autologin. Contrary to its general purpose name, it works on LightDM greeter "slick-greeter" only.

Some further comments on the MR:

• Autologin is now disabled by default. It can be enabled by the user in the administration/settings GUI "Login Window", tab "Users" section "Automatic login".
• The virtual keyboard on the greeter is enabled by default. It can be disabled in the greeters top bar accessibility menu.
  • The onboard layout can be changed in the administration/settings GUI "Login Window", tab "Settings" section "Panel indicators".
  • On installations older than July 2024 the virtual keyboard will show up again after reboot. This is because on these installations the home directory of user "lightdm" is at /var/run/lightdm, which is not consistant over boots. End of June 2024 Alpine MR https://gitlab.alpinelinux.org/alpine/aports/-/merge_requests/68087 changed this to home directory /var/lib/lightdm. However, this applies only to new installations since then. Changing the home directory of user "lightdm" on runtime cannot easily be done by post-upgrade script. Affected users would need to logout, stop lightdm service, change the home directory of user "lightdm" and start lightdm service again. Or alternatively just enable autologin to avoid "seeing" the greeter with the virtual keyboard.
• The option "draw-user-backgrounds" doesn't work in MATE (https://github.com/mate-desktop/mate-settings-daemon/issues/170). Therefore on MATE it's necessary to set a background picture path in the the slick-greeter gschema.override file.
• There are two separate maintanance commits. One removes a line about tinydm from the post-install scritp. The other cleans up the package() section of Xfce4.