systemd: try to use socket activation for i3

Unfortunately, this does not work as not everything is using
I3SOCK (notably `i3 --get-socketpath` does not).

dotfiles/systemd/i3-companion.service

@@ -1,6 +1,7 @@
 [Unit]
 Description=i3 companion
 PartOf=graphical-session.target
+Requires=i3.socket
 
 [Service]
 ExecStart=%h/.config/i3/bin/i3-companion

dotfiles/systemd/i3.service

@@ -6,9 +6,6 @@ After=picom.service
 After=xsettingsd.service
 After=tmux.service
 After=ssh-agent.service
-Before=wallpaper.service
-Before=polybar.service
-Before=i3-companion.service
 
 [Service]
 ExecStart=/usr/bin/i3

dotfiles/systemd/i3.socket

@@ -0,0 +1,7 @@
+[Unit]
+Description=i3 window manager
+PartOf=graphical-session.target
+
+[Socket]
+ListenStream=%t/i3.sock
+ExecStartPost=/bin/systemctl --user set-environment I3SOCK=%t/i3.sock

dotfiles/systemd/polybar.service

@@ -1,6 +1,7 @@
 [Unit]
 Description=Status bar for i3
 PartOf=graphical-session.target
+Requires=i3.socket
 
 [Service]
 ExecStart=%h/.config/i3/bin/polybar

dotfiles/systemd/xsession.target

@@ -3,7 +3,6 @@ Description=X session
 BindsTo=graphical-session.target
 Wants=autorandr.service
 Wants=dunst.socket
-Wants=i3.service
 Wants=i3-companion.service
 Wants=inputplug.service
 Wants=misc-x.service