\note-by-number supports flag-styles (issue 7231072)

[thomasmorley65]

Reviewers: ,

Message:
Please review.

The regtests markup-note.ly and markup-note-styles.ly are modified.
Changes are expected.

Description:
\note-by-number supports flag-styles

Issue 3104

For mensural and neomensural the flag is changed due to the
note-head-style.
Straight flags are possible using the new introduced flag-style-property

Please review this at https://codereview.appspot.com/7231072/

Affected files:
  M input/regression/markup-note-styles.ly
  M input/regression/markup-note.ly
  M scm/define-markup-commands.scm