Release: bump VERSION_DEVEL.
PO: update template.
Release: update news.
Release: bump VERSION.
Issue 3449: Shift accordion register symbols down by half a line width

This makes the documented proposals for combining the accordion
register symbols together with the accordion dot symbol match in
positioning.  It would have been feasible instead to shift the
(centered) accordion dot, but some register symbols work with
different line widths.

The net result will now exhibit a slight descender (namely half a
linewidth) of the accordion register symbols when used in a running
line.
Issue 3450: \addlyrics doesn't work with TabStaff

More details about limitation of \addlyrics in
Documentation/notation/vocal.itely

Add a hint, that \addlyrics does not work with TabStaff

Signed-off-by: David Kastrup <dak@gnu.org>
Issue 3151: Fix some oversights of \layout in \book

follow-up for issue 3151

Signed-off-by: David Kastrup <dak@gnu.org>
Doc (NR, repeats): Minor clarification.
Issue 3455: Fix flex warning: -s option given but default rule can be matched
Remove obsolete Staff.minimumVerticalExtent unsetting in input/regression/system-extents.ly
input/regression/tuplet-no-stems.ly: fix obsolete number-visibility
Issue 3444: No warning for misspelled properties

This makes sure that properties are checked to be assignable even when
assigning #f or '() to them.