Add detection support for legacy (Gnome) Cosmic DE

[demurgos]

As reported in #13, there is no value configuretly for XDG_CURRENT_DESKTOP to match the legacy (Gnome) Cosmic DE.

Quoting from the PR:

On the current alpha for COSMIC Epoch, XDG_CURRENT_DESKTOP appears to be set to COSMIC, vs the legacy GNOME-based version of COSMIC where it appears to be set to pop:GNOME (on Pop!_OS at least). Since this is still an alpha, I'm not sure if that will change in the future, but this should work for now I think.

Also, unless I missed it, I didn't see an actual check for the legacy GNOME-based COSMIC. I didn't bother adding one since I'm not sure if it's different enough from base GNOME to warrant a separate check, but I thought I'd at least mention it.

We should double-check the right value and add support for it.