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.
Tabs now align to tab stops instead of always spanning 8 spaces when
show_whitespaces is added as a highlighter.
This fixes issue #1453.
A regression test is also provided.