Piano pedalling patch

[Chris Jackson]

I've been hacking around with piano pedalling, to try to implement
bracketed pedals, and my patch is now ready to be tested.  

Firstly, all pedal symbols (Ped. *  texts, and brackets) are now
horizontally aligned, using a line spanner. I think this looks much
neater than having them attached to notes. 

To get the brackets, you set 
\property Staff.SustainPedal \override #'pedal-type = #'bracket
to obtain pedal markings which look like

|________/\_______/\__| 

Also there's 
\property Staff.SustainPedal \override #'pedal-type = #'mixed

to get

Ped. ______/\_______|

which is what the Stone book recommends, as it's easier to follow when
there's more than one sort of pedal occuring simultaneously. You can do
similar things for Sostenuto and UnaCorda pedals. 

As a result, the piano pedal engraver is now a lot hairier, a bit like
the dynamic engraver.  The actual dimensions of the brackets are set
using a new function in text-spanner.cc. Please let me know how much I
have coded things in the proper lilypond way! 

-- 
chris

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