A receptacle is an unfocusable empty leaf node.
Receptacles are used for building a tree without creating windows.
Example:
bspc node -i
bspc node @/ -p east -i
bspc node @/2 -p north -i
bspc rule -a Abc:abc -o node=@/1
bspc rule -a Ijk:ijk -o node=@/2/1
bspc rule -a Xyz:xyz -o node=@/2/2
Fixes#259.
There's no constraints on desktop and monitor names, therefore, using a
desktop or monitor name as descriptor is ambiguous.
We put an end to this ambiguity by introducing desktop and monitor IDs.
`bspc query -{M,D}` now yields IDs instead of names.
Fixes#397.
