X: Improve lockscreen handling

Added lockscreen trapping and media pausing for lockscreen.
When it is invoked `lockscreen`, all media will be attempted to be pause
and the system will be muted.
Then the lockscreen is engaged.
This should happen automatically when the screen shuts off or the system
suspends or hibernates and can be invoked manually, as before, with
super+x shortcut.

i3/.config/i3/config

@@ -180,12 +180,6 @@ bindsym $mod+F12 reload
 # restart i3 inplace (preserves your layout/session, can be used to upgrade i3)
 bindsym $mod+Shift+F12 restart
 
-# set a pretty wallpaper
-exec_always --no-startup-id feh --bg-scale ~/media/pictures/wall.jpg
-
-# launch polybar (script ensures only 1 instance existing at a time)
-exec_always --no-startup-id polybar-launch top
-
 # default workspaces for most used apps
 # assign [class="^qutebrowser$"] → number 1
 # spotify needs for_window, see https://i3wm.org/docs/userguide.html#assign_workspace
@@ -205,3 +199,7 @@ for_window [class="scratchpad"] move scratchpad
 bindsym $mod+m exec i3-input -F 'mark %s' -l 1 -P 'Mark: '
 # read 1 character and go to the window with the character
 bindsym $mod+apostrophe exec i3-input -F '[con_mark="%s"] focus' -l 1 -P 'Goto: '
+
+# launch polybar (script ensures only 1 instance existing at a time)
+exec_always --no-startup-id polybar-launch top
+