Add ly:staff-symbol-line-thickness Scheme-callback, returning line thickness

Add ly:staff-symbol-line-thickness Scheme-callback, returning line thickness

-) Add a callback ly:staff-symbol-line-thickness in staff-symbol-referencer-scheme,
   which simply gives you a Scheme interface to
       Staff_symbol_referencer::line_thickness (g)
-) Use this to shift inverted flags in the flags-in-scheme test case
   by the stem-width (the 'thickness grob property of the stem is given
   in units of the line-thickness!), so the flags really attach to the
   left edge of the stem