Add output-attributes grob property to replace id (issue 308430043 by address@hidden)

[paulwmorris]

Reviewers: ,

Message:
Please review, thanks, -Paul

Description:
Add output-attributes grob property to replace id

Allows setting multiple attributes on group
nodes (<g> in SVG output), specified as an alist.

Please review this at https://codereview.appspot.com/308430043/

Affected files (+84, -35 lines):
  M Documentation/changes.tely
  D input/regression/id.ly
  A input/regression/output-attributes.ly
  M lily/grob.cc
  M lily/stencil-integral.cc
  M lily/stencil-interpret.cc
  M python/convertrules.py
  M scm/define-grob-properties.scm
  M scm/define-stencil-commands.scm
  M scm/output-ps.scm
  M scm/output-svg.scm