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 https://github.com/slgobinath/SafeEyes/pull/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).