Cleaned up LilyPondFile list properties.

These properties are now read-only instead of read / write:

    * LilyPondFile.file_initial_system_comments
    * LilyPondFile.file_initial_system_includes
    * LilyPondFile.file_initial_user_comments
    * LilyPondFile.file_initial_user_includes

All four properties implement lists. No reason for the properties to be read /
write.