gtkgreet instead of Tuigreet

[Sunderland93]

gtkgreet is more like Gnome's GDM, it looks quite similar and support theming through CSS. Only issue is it needed separate Wayland compositor to run (upstream recommends Cage). I've push initial config to https://github.com/Ubuntu-Sway/iso-builder/blob/lunar/etc/config/includes.chroot/etc/greetd/config.toml , I can test when PPA is rebuild for Lunar Lobster (I'll start to work on it next week).

Right now switch to gtkgreet is blocked by three major problems:

• [ ] 1) running gtkgreet in cage is extremely slow (probably due to lack support for wlr-layer-shell in cage, which makes gtkgreet use xdg-shell instead). The first launch is slow, but subsequent ones are instantaneous
• [ ] 2) gtkgreet doesn't support session files, so it can't parse /usr/share/wayland-sessions for available sessions. Those need to add in /etc/greetd/environments, which makes dificuilt to add something like sway --unsupported-gpu, because Calamares can only add current desktop name (e.g. sway)
• [x] 3) gtkgreet needs it's own CSS file for theming, which in turn also need to added by Calamares (it should add -s /etc/greetd/gtkgreet.css command line to default_session cmd)

[Sunderland93]

So for now, I revert config.toml to use greetd-tuigreet, until all issues with gtkgreet is resolved.