Some highlighters, such as wrap or line numbers, are not intended
to be used multiple times on the same display. Add support for unique
ids that are used by highlighters to disable themselves if another
unique highlighter with the same id is supposed to override them.
The usual highlighter "precedence" takes, place, that it, that most
nested highlighter will the the one to run (window in priority to
buffer in priority to global).
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.
Highlighters now run in 3 phases:
Wrap, Move, and Colorize. That way we guarantee the wrap
highlighter runs first, then eventual line numbers/flags,
and finally the colorizers.
We also run a `compute_display_setup` method thats responsible
for computing the lines that will be displayed, eventually
scrolling the view to ensure the cursor is visible.
