Suggest setting XDG_CURRENT_DESKTOP if DE cannot be detected

[greenfoo]

First of all, let me say this is a stupid PR, so feel free to ignore it if you want :)

Now, some context: in sway, you are responsible for setting environment variable "XDG_CURRENT_DESKTOP=sway" before running it.

The "problem" is that if you are using a logging manager such as GDM, you need to manually edit the "/usr/share/wayland-sessions/sway.desktop" which is overwritten every time sway is updated (at least in Arch), which happens every few months.

Well... this is the third time this has happened to me and each time I completely forgot what the issue was and had to compile a new version, attach the debugger and step through the code to figure out what the problem was (yes... I must have the worst short term memory in the world).

Anyway, in this PR I simply add a message suggesting to check environment variable XDG_CURRENT_DESKTOP when the desktop environment cannot be detected in the hope that this will save me (and probably others) ~1 hour in a few months once sway is updated again.

Thanks!

[mmahmoudian]

Shouldn't the env vars be set somewhere other than .desktop file?

https://github.com/swaywm/sway/wiki/Setting-Environmental-Variables

[borgmanJeremy]

Thank you!

[greenfoo]

Shouldn't the env vars be set somewhere other than .desktop file?

https://github.com/swaywm/sway/wiki/Setting-Environmental-Variables

Thanks! I had no idea how to set environment variables for gdm (which is a systemd service). Turns out "man environment.d" is exactly what I was looking for :)