sxhkd: Add bspc hidden flag functionality

Note that window order is not based on the order in which windows are
hidden, but something else entirely. For this reason, it is recommended
to hide only one window at once.

A more robust solution would be to let the user choose from a list of
available hidden windows, potentially through rofi's dmenu capability.

sxhkd/.config/sxhkd/sxhkdrc

@@ -55,6 +55,10 @@ super + {_,shift} + n
 super + {x,y}
     bspc node -g {sticky,private}
 
+# FLAGS: Push and pop windows from the hidden stack
+super + {_,shift} + b
+  bspc node {-g hidden,$(bspc query -N -n .hidden | tail -n 1) -g hidden=off}
+
 # STATES: Toggle between node states
 super + {t,r,f,space}
     bspc node focused -t ~{tiled,pseudo_tiled,fullscreen,floating}