Make fences visible

This feature is meant to lift the ambiguity that exists in many simple situations regarding the structure of the underlying tree.

In the following example:

There are two possible underlying tree:

The same remark applies to this scenario:

Where the fenced rendering are:

The color of a fence is related to its tree depth.

How about drawing a border of a fence. Edit: After drawing a demo I think it'll get too messy

In the mean time, before this ships (which i'm looking forward to btw) I use these key bindings for discovering what the tree structure looks like.

super + alt + p
  bspc node -f @parent
super + alt + {f,l}
  bspc node -f {any,last}.descendant_of.leaf

super + alt + p (p for parent) selects/focuses/highlights the parent node, you can keep pressing it to highlight the parent of the currently highlighted parent node super + alt + f (f for first node) selects/focuses the first node of the top highlighted parent node, super + alt + l (l for last) focuses the window/node that was last focused before a parent node was highlighted/selected/focused.