Implement make-bow-stencil, make-tie-stencil for use in markup-commands undertie and overtie (issue 270640043 by address@hidden)

[thomasmorley65]

Reviewers: ,

Message:
please review

Description:
Implement make-bow-stencil, make-tie-stencil for use in markup-commands
undertie and overtie

issue 3088

In a follow up it is planned to replace make-parenthesis-stencil with
an appropiate setting of make-bow-stencil and to partially rework the
parenthesize-markup-command

Please review this at https://codereview.appspot.com/270640043/

Affected files (+211, -0 lines):
  M Documentation/changes.tely
  M scm/define-markup-commands.scm
  M scm/stencil.scm