Fix unproper nesting of various property overrides

Fix unproper nesting of various property overrides

Due to the unavailability of \temporary\override, \acciaccatura and
\slashedGrace had been losing any previous setting of
Flag.stroke-style.

\crossStaff had been losing the previous setting of Flag.style.

\harmonicByFret and \harmonicDots had been losing the previous setting
of TabNoteHead.stencil, NoteHead.Y-extent and NoteHead.stencil.

\temporary\override is employed in this patch for fixing those
shortcomings.