[Feat] added display manager entry

[samuelskovbakke]

I have added a display manager entry. So users can now add displaymanager to get something like:

Display Manager: SDDM

[areofyl]

This only works on systemd, which is a problem since fetch runs on non-systemd distros too (prime example is Gentoo). Could you detect the DM by scanning running processes (like we do for WM) instead? Also the substring search on the service file could false-positive on comments or paths: matching the ExecStart= line specifically would be more reliable. And it probably shouldn't be in the default field list since many tiling WM users don't use a DM.

[samuelskovbakke]

I have changed it to process scanning instead, but on systems that have a lot of running processes I think this is quite slow. Maybe we should rather check if the distro is running systemd and do the ExecStart= approach and only do process scanning if systemd is absent?

[areofyl]

Looks a lot better!