Release: bump VERSION.
Release: update news.
PO: update template.
python/convertrules.py: bump version for 2.18 release
Adds outside-staff-interface and outside-staff-axis-group-interface

When a property exists in an interface, the property should make sense for
a sensible of grobs that implement that interface.  Otherwise, the property
should be part of a separate interface that applies to a sensibly-sized
group of grobs.

Various properties having to do with outside-staff calculations belong to
grob-interface.  This does not make sense for many grobs (StaffSpacing,
NoteSpacing, SpacingSpanner, StaffSymbol just to name a few).  There is a
limited collection of grobs for which outside-staff properties make sense.
The outside-staff-interface provides a separate interface to be implemented
by these grobs.

The same is true for axis groups that place outside-staff grobs, which form
a limited subset of grobs implementing the axis-group-interface.  Rather than
putting the relevant properties in the axis-group-interface, we move
them to an outside-staff-axis-group-interface.
Doc: Extending addition about calling stencil func

Issue 3669

Came out of:
http://lists.gnu.org/archive/html/lilypond-user/2013-11/msg00587.html

Text based on Keith OHara's suggestions

Indicate that it is possible to call the stencil function in a user's
own override of the stencil property
Doc: Tidy up of chord-names-jazz.ly

Added a new context definitigon to remove the Bar Number Engraver
as it was clashing with the instrument names and looked messy.
As it was not actually pertinent to the example I thought this the
simplest method

Also tidied up the spacing of the information as per standard doc
guidelines
Doc: NR - Tidy up of 3.5.1 and 3.5.3 - MIDI

Seemed some of this information was repeeated (and so redundant)
with additional grammar and sentence structure changes and more consistent
@examples.

No new technical information added.
Doc: NR Appendix A10.3 \path examples

Issue 3587

Added two extra examples for the \path command

Also moved two paras down as it seemed more of a logical order
Doc: NR Updated Delayed Turn Snippet

Issue 3369

Suggestion from Arno Waschk

Simplification of original snippet

Note: This snippet will only work since 2.17 versions (where the /single
command has been implemented) and does not work on the 'current' LSR
(which is still at 2.14 as of this patch).
Run makelsr.py

added new snippet - commit c9acfc85c
Issue 621: Dynamics should avoid cross-staff BarLines
(e.g. GrandStaff, PianoStaff etc)

Dynamics usually have extra-spacing-width set to an empty interval so
that their placement does not cause other elements to shift.

With span bars, however, the resulting overlap is a worse cure than
the problem.  So this switches off the width-hiding setting of
extra-spacing-width inside of staff groups using span bars by default.
No extra space is allocated, so dynamics will clear the span bars only
narrowly, a reasonable compromise.
Issue 3720: Doc: Built-in templates for SATB vocal scores

  Add documentation to LM Templates Appendix

  Add cross-refs from other parts of the manual set
Issue 3720: Add SATB built-in template

Signed-off-by: Trevor Daniels <t.daniels@treda.co.uk>
Issue 3635: Doc LM: Improve the explanation of 'force-hshift

  Add an explanation of the 'horizontal-shift property

  Expand the explanation of the 'force-hshift property

  Make the example of using the 'force-hshift property
  more transparently clear.
ledger-lines: horizontal space only when present
spacing-options: clarify documentation
springs: comment and typo
note-spacing: stretch somewhat uniformly

Start with a basic spring based on the note duration, and apply optical
corrections to that.  This results in more consistent springs thus more
uniform stretching in polyphonic situations.
note-spacing: let compressibility be uniform; issue 3304

Comments implied that half the width of accidentals, etc., was added
to the ideal spacing between notes, but in fact only compressibility
was affected.  The non-uniform compressibility caused note-spacing
to become non-uniform when the lines were compressed.  For example
A sequence {\stemUp a a d a] would have the head of the D tuck under
the preceding A.

This commit keeps spacing uniform on compressed lines until objects
come within padding of each other.