Add the command \offset to LilyPond (issue 8647044)

[david . nalesnik]

Reviewers: ,

Message:
Please review.

Thanks,
David

Description:
Add the command \offset to LilyPond

The ability to offset default values of various properties would be a
useful enhancement of LilyPond.  Currently, this is possible for the
property 'control-points using the \shape command.  The following patch
seeks to generalize the application of offsets to grob properties.  Both
overrides and tweaks are supported, on the model of recent revisions to
\shape and \alterBroken.

Offsets are currently limited to three data types: number, number-pair,
and number-pair-list (the latter is defined by this patch and represents
the type used, for example, by 'control-points).  Offsets are limited to
immutable grob properties.  (These are listed in
`scm/define-grobs.scm'.)
Offsets are reckoned against default values.  It is not possible to
accumulate offsets, nor to offset against a user-defined procedure.

Please review this at https://codereview.appspot.com/8647044/

Affected files:
  A input/regression/offsets.ly
  M ly/music-functions-init.ly
  M scm/c++.scm
  M scm/music-functions.scm