Prefix generated source and pdf files with lily-

TeX distributions require not to introduce name clashes.
Therefore I have to prefix all files that may end up in the LaTeX
or system search path with lily-

TODO: Update commands.
TODO: Update script
Prefix lily- in LaTeX commands

insert a prefix in the \lilyImage command inside the existing predefined commands.

TODO: Update script to automatically handle this:
- if command does _not_ start with 'lily-" prefix the generated source file
- pdf will automatically be correct
- if command does _not_ start with 'lily-"
  prefix the file reference in the \lilyImage call
genImageCommands: prefix file name with lily-

Generated source file (.ly) and pdf are prefixed with lily-
Argument to \lilyImage too
Add fancy image command example

Test for the preceding modification to the genImageCommands script