Add measure_separator deprules

[hajicj]

The measure_separator class also contains the repeat (so that the repeats are not double-layered).

• [x] separator to barlines
• [x] separator to fermatas and such

[hajicj]

Note: repeats are a different kind of measure separator, they are at the same level of abstraction.

[hajicj]

Note: we need to handle symbols at the beginning of the system (e.g., bracket + barline).

Should there be a different separator? There probably should, because it's not really a measure delimiter... for instance, it is duplicated with the last barline of the previous line.

What about: system_start_delimiter

[hajicj]

Problem: repeats cannot be measure_separators, because back-to-back repeats are two repeat symbols, but should only be one measure_separator, as they are just one boundary between bars.

[hajicj]

The system start symbol could maybe be better represented by a hierarchy of the already present staff_grouping symbol.

[hajicj]

Done.