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.
Fixes#1433 although in a slightly different way than requested:
We ensure that scrolloff *displayed* lines are visible below the
cursor, not scrolloff *buffer* lines.
