Support for LilyPond 2.20.0

[Richard Shann]

There is a new release of LilyPond out (this happens every few years).
Most Denemo scores continue to typeset just fine if you install this
version, but Playback View will not work and Book Titles and Global
Transpose are only working because the new version is tolerant of old
syntax.

So I've implemented changes to Denemo that allows it to fully support
two versions of LilyPond at any one time - the current stable and the
last stable version.
This version of Denemo was built overnight, but the Windows built last
night still has LilyPond 2.18 built in - you can install 2.20 and
change your preferences to point to the lilypond-windows.exe of that
version (it would be at C:\Program Files\lilypond\usr\bin\lilypond-
windows.exe by default) if you want to try it out.

Richard