Issue 3999: Make tablature half-note stem-spacing adjustable.

Issue 3999: Make tablature half-note stem-spacing adjustable.

This adds a new property to the Stem grob called
'double-stem-separation (default=0.5), that allows users to adjust
the space between the double-stemmed half-notes in tablature:

\new TabStaff {
  \tabFullNotation
  c4 c2 c4
  \override Stem.double-stem-separation = 0.3
  c4 c2 c4
}

It also centers the stems on the fret number and adjusts the
X-extent accordingly.