\lilypondMFE:implement system delimiter

[uliska]

Add an option to place something between systems in a lilypond multifile example (like with the output of lilypond-book).

It should be realized as some similar variables in the package: Initially set it to some value (e.g. a line break) and offer a command to change it.

Ideally one would have different items for different contexts:

• first/last/other system changes
• at top or bottom of page