Instead of triming only buffer ranges, add a trim_from method to
display line to keep the initial N columns, we know how many columns
are used by non-trimable widgets in DisplaySetup::widget_columns so
we can just pass this.
Also restore the previous logic for face merging
Fixes#4670
Make the column highlighter faces final, and change final logic to
give precedence to the base face when both the base and new face are
final.
Fixes#4669
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
Final is more granular, it consists of FinalFg (f), FinalBg (g)
and FinalAttr (a) which control if a face's fg, bg, or attributes
fully overwrite the previous face (instead of merging) and if
following faces apply on top of this face or not.
Fixes#2388 if the Whitespace face has the FinalFg flag.
