Color and/or parenthesize single dots in fret-diagrams

Color and/or parenthesize single dots in fret-diagrams

Issue 2752

Makes it possible to color and/or parenthesize single dots in
fret-diagram-verbose

Introducing two properties for use in fret-diagram-details
 - fret-label-horizontal-offset
    affecting the fret-label-indication, like the existing
    `fret-label-vertical-offset'
 - paren-padding
    affecting the padding of the parenthesis

Extending relevant reg-tests.

Documenting it in NR, fretted-strings.itely