enhance lilypond path preference initializer

[nittka]

Currently, there are three default installation paths in CompilerPreferenceInitializer, each for one operating system. I usually install software to a non-default location, so applying defaults on the compiler preference page will always destroy the lilypond path.

I suggest modifying the initializer in the following way:

• check for a start parameter (analogous to the unit test start parameter -Dlilypond.path)
• check for the Oomph user setup variable "lilypondlocation" (as used in the Elysium Oomph setup)
• if none of these exist, fall back to the current implementation