Use standardized environment variable for desktop detection

[deltragon]

Currently, we (mostly) use DESKTOP_SESSION - however, that seems to be something that only happened to be supported, but never actually standardized. (Eg. sway already doesn't support it, for it we already use XDG_CURRENT_DESKTOP.)
Additionally, this non-standardization leads to issues like #546 - where distros just change DESKTOP_SESSION as they see fit.

There are two options here:

• XDG_CURRENT_DESKTOP, standardized by the desktop entry spec, allows multiple values
• XDG_SESSION_DESKTOP, set by systemd, only a single value

It seems slightly preferable to use XDG_CURRENT_DESKTOP, as it is more standard. However, then we need to handle the multi-value case (probably best to just use the first value we recognize).

[archisman-panigrahi]

I am running Kubuntu 24.04 (Plasma 5.27 with Wayland), and echo $XDG_CURRENT_SESSION generates an empty output

[archisman-panigrahi]

XDG_CURRENT_DESKTOP is KDE, but XDG_DESKTOP_SESSION is blank.

$ echo $XDG_CURRENT_DESKTOP
KDE
$ echo $XDG_DESKTOP_SESSION

[deltragon]

I typoed it - it's supposed to be XDG_SESSION_DESKTOP

[deltragon]

FTR, on Fedora 41/Plasma 6 Wayland I get:

DESKTOP_SESSION=plasma
XDG_SESSION_DESKTOP=KDE
XDG_CURRENT_DESKTOP=KDE