Add original-breaks.ly commands (issue 150670043 by address@hidden)

[lilyliska]

Reviewers: ,

Description:
Add original-breaks.ly commands

The commands defined in this patch allow the user to insert breaks
(line and page) from the original manuscript that are conditionally
respected or discarded. This is very useful for entering and
proof-reading music from larger scores.

Actually I would prefer having them enabled automatically (i.e.
parsed on initialisation without requiring to include the file
explicitly. But I think the way it is done in the patch will make
it easier to accept its inclusion.

Two commits:

Add docs for original-breaks.ly

Add original-breaks.ly

Define commands:
- origBreak
- origPageBreak

Depending on the presence and state of variables
- keep-original-breaks
- keep-original-page-breaks

these result in a \break or \pageBreak or nothing.
Default is #t (so when the file is included original
breaks are automatically respected).

Additionally define top-level commands
- \keepOriginalBreaks
- \discardOriginalBreaks
- \keepOriginalPageBreaks
- \discardOriginalPageBreaks

The idea is to let the user enter breaks from the manuscript
she is copying. While working on the content of the score
it is very handy to have LilyPond compile the score with
the breaks of the original manuscript, but of course one
doesn't want to enter hard-coded breaks.

Please review this at https://codereview.appspot.com/150670043/

Affected files (+148, -0 lines):
  M Documentation/notation/spacing.itely
  A ly/original-breaks.ly