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
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
