alignmentX attributes removed.

alignmentX attributes, which used to be
written to InputChordSymbol, RestSymbol
and OutputChordSymbol, are no longer
written anywhere in score files.
Instead, InputChordSymbol writes its
msDuration to a new inputChord:msDuration
attribute. (The msDuration is known, and has
already been used to find the chord's
duration class.)
The Assistant Performer will now use the
common msPosition to link InputChords
to Seqs. This is much safer, and leads to
more elegant code.
Moved InputControls inside InputChordDef

This means that the output voices in a score
are identical, whether or not there are any
input voices in the score.
This leads to simpler, more elegant code.
(Less need for consistency checking.)
Also revised and updated the new
svgScoreExtensions document, and did
some refactoring.
The "score:"prefix is now only written for
attributes and elements in the score when
neccessary.
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