use X-parents to align MMRNumbers, MMRTexts and PercentRepeatCounters

It makes more sense to use X-parents than Y-parents for horizontal
alignment. To be able to do this, we have to ensure that X-parents of
MultiMeasureRestNumber, MultiMeasureRestText and PercentRepeatCounter
won't be overwritten by set_bound (these grobs are spanners).  We can
do this because parents of these grobs are spanners as well.

Expected changes in output: none.
Authors.itexi - Updated Developers list

Issue 3964

Added/updated to Devs Current, Translations and Core Current.

Based on checkins since Feb 2014 until June 2014
Doc: NR Section 4.3 - reformatting as per CG Guidelines

Start of issue 3960

This section does not adhere to  many aspects of the Contributors'
Guide Documentation policies.

This specific patch therefore is the 'first stage' of this issue
so as to clean up the section before then making changes to
the technical information. There is too much to do for a single patch,
so I expect to have to make a few more additional patches to tidy this
section up completely before the issue can be properly closed.

This patch includes:

* Correct Line lengths

* Added @seealsos where appropriate

* Re-ordered some of the @nodes so that they are grouped more logically

* Moved @subsections to @unnumberedsubsubsecs:
A few of these sections seem to 'talk through the code' so may
need to be improved later on. However, for now I have moved these
subsections to unnumberedsubsubsec under the main 'Page Breaking'
node. Moved one of these new unnumberedsubsucsecs around so that
the sections on the different breaks come before the turning one.

* Created new Snippet:
One subsection was really a glorified snippet so I have made it into
one.
Makelsr.py run

For previous commit aec840a735b33a4be38b10baeb40f8e660e3d55c
Doc: Usage: Added a clarifying paragraph about Error Messages

Issue 3873

Added a clarifying paragraph explaining that (the same)
diagnostics message can occur in multiple places for
(the same) input depending on the type and its context.
Adding new snippet generated from previous Makelsr

See commit 499ff80772530b66e59768a4251bde08bd77fab4

My mistake. Sorry.
Issue 3966: Revert "Allows the user to override the text property of ChordName"

This reverts commit 0d4c79ec0f5d728ac7fe58b87ccf51079ec33766.

Conflicts:
	input/regression/chord-name-override-text.ly
	lily/chord-name-engraver.cc

Reverts patch of issue 2813.
Issue 3966/2: Allows the user to override the text property of ChordName

This is a reimplementation of issue 2813.