Always start with full buffer lines and trim the display buffer at
the very end, treat non-range display atoms as non-trimable in that
case and keep track of how many columns are occupied by "widgets"
such as line numbers or flags.
Fixes#4659
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.
