set-face now takes a scope argument, and faces can be overridden on
a buffer or window basis.
colorscheme apply on global scope, which should be good enough for
now.
Fixes#1411
That means we can now have highlighters active at global, buffer, and
window scope. The add-highlighter and remove-highlighter syntax changed
to take the parent path (scope/group/...) as a mandatory argument,
superseeding the previous -group switch.
