Replace Slock with Multilockscreen

- Add xss-lock background process to dwm autostart
- Change lock command in dwm and polybar to loginctl lock-session
- Add config file for multilockscreen
- Remove slock submodule

.gitmodules

@@ -10,6 +10,3 @@
 [submodule ".config/polybar"]
 	path = .config/polybar
 	url = git@github.com:balajsra/polybar.git
-[submodule ".config/slock"]
-	path = .config/slock
-	url = git@github.com:balajsra/slock.git