Add support for a third color in face definition that controls
the underline and a 'c' attribute for curly underline (that takes
precedence over 'u' if both are specified)
Allow empty colors to mean default, so that `,,red+u` means the
same as `default,default,red+u`
Fixes#4138
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.
Support arbitrary orders for column highlighters (it was previously
failing when column highlighters were not applied in column order).
Fix show_matching tab handling at the same time (horizontal scrolling,
tab characters and show_matching were behaving badly).
Window highlighting now runs user highlighters, then built-ins for each
phases, instead of running all phases for user highlighters, then all
phases for built-ins.
We now consider unprintable character to be 1-column width as we know
we will display them as "�".
Fixes#1615Fixes#1023
