add support files for cosmic-session

data/cosmic-comp.service

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+[Unit]
+Description=Cosmic wayland compositor
+BindsTo=cosmic-session.target
+Wants=cosmic-session-pre.target
+After=cosmic-session-pre.target
+Before=cosmic-session.target
+
+[Service]
+Type=simple
+ExecStart=/usr/bin/cosmic-comp
+Restart=never
+ExecStopPost=/usr/bin/systemctl --user unset-environment DISPLAY WAYLAND_DISPLAY

data/cosmic-service

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+#!/bin/bash
+
+# From: https://people.debian.org/~mpitt/systemd.conf-2016-graphical-session.pdf
+
+# robustness: if the previous graphical session left some failed units,
+# reset them so that they don't break this startup
+for unit in $(systemctl --user --no-legend --state=failed --plain list-units | cut -f1 -d' '); do
+	partof="$(systemctl --user show -p PartOf --value "$unit")"
+	for target in cosmic-session.target graphical-session.target; do
+		if [ "$partof" = "$target" ]; then
+			systemctl --user reset-failed "$unit"
+			break
+		fi
+	done
+done
+
+# /etc/profile contains a lot of important environment variables
+source /etc/profile
+
+export XDG_CURRENT_DESKTOP=cosmic
+
+# save environment variables that will be added to systemd
+new_env=$(systemctl --user show-environment | cut -d'=' -f 1 | sort | comm -13 - <(env | cut -d'=' -f 1 | sort))
+
+# import environment variables from the login manager
+systemctl --user import-environment
+
+# then start the service
+systemctl --wait --user start cosmic-comp.service
+
+# cleanup environment
+systemctl --user unset-environment $new_env

data/cosmic-session-pre.target

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+[Unit]
+Description=Cosmic session early services
+Documentation=man:systemd.special(7)
+RefuseManualStart=yes
+StopWhenUnneeded=yes
+BindsTo=graphical-session-pre.target
+Before=graphical-session-pre.target
+Before=cosmic-session.target

data/cosmic-session.target

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+[Unit]
+Description=Cosmic session
+Documentation=man:systemd.special(7)
+RefuseManualStart=yes
+StopWhenUnneeded=yes
+BindsTo=graphical-session.target
+Before=graphical-session.target

data/cosmic.desktop

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+[Desktop Entry]
+Name=Cosmic
+Commment=This session logs you into Cosmic
+Exec=/usr/bin/cosmic-service
+Type=Application
+DesktopNames=pop:COSMIC
+X-GDM-SessionRegisters=false