A common pattern is for info/echo messages to be generated by idle
hooks but the clearing of previous info/echo was done immediately on
normal mode events. This led to flickering of the info box especially
when a hook was repeatidly generating the same info (like moving
a cursor in the same word where the hook reacts to the word under
the cursor).

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

This likely has lots of rough edges, but should be an initial proof
of concept to support folding.