FR: Replace "Session:" label with a session type selector (Wayland, X11 etc)

[max-ishere]

Is this something anyone actually wants? Not theoretically, I need this theoretically-just-in-case too, practically is this a thing you want?

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Currently regreet has 2 options for selecting a session presented as a dropdown (X11 & Wayland combined) with a secret fallback to the login shell if there are no XDG sessions, and an arbitrary command. This works well enough, but what if the user doesn't have a default shell or they specifically want that, or what if they wanna choose a different shell from /etc/shells.

This is just an idea, the "Session:" label is replaced with a dropdown that has the following options (ordered by priority, default on startup is the first one):

	Source
Wayland	$XDG_DATA_DIRS/wayland-sessions
X11	$XDG_DATA_DIRS/xsessions
Default shell	/etc/passwd
Shells	/etc/shells

If any of the categories were not found during loading, the option for that would be not be present.

The only issue I can anticipate is with the default shell option. If at least 1 user has it, the option would show up, but then there should be code to handle the username (regardless of it coming from the dropdown or the manual input) not having a default shell.

Handling it is not hard, just stop the user from starting auth or if the UI would in the future allow the user to change the session during auth, just put them in a state like the image below (show notification + not make it look like the auth was discarded)

At the IPC level this is the state between the last auth response (greetd returned suceess) and before start session that actually specifies the command and the ENVs.

Yes, this UI is not in this repo, its from my fork as part of #85, eventually it will be proposed as a set of PRs, I am slowly working on that.

[max-ishere]

I forgot to mention how this came into my head, I was thinking about how in #65 there is an edge case where some X11 sessions start the x server and some don't and how this could be addressed by having a separate UI for X sessions specifically, or how something like Gnome might have an X11 and Wayland modes and right now its up to the packagers of the desktop files to make sure those are actually possible to distinguish (xdg file ID being unique + including the type in the Name attribute.

These are not issues I have with 1 user and 1 session, so I dont know how relevant it is.

I can't imagine someone using the GUI greeter to login into a shell, but oh well, it could actually be a thing.