Engrave window: bar numbers, and two silent LilyPond faults

A system can now be given the measure it starts at, in a First bar field
beside key and time. Per slice, since it is the one thing about a
replacement that cannot be inherited from the song. Bar numbering is a
Score property, so it is set once on the first staff, and made visible
only at a line beginning — that vector is fussy: #(#f #t #t) also prints
a number mid-system and #(#f #t #f) prints the second bar's rather than
the first's. It travels in the bundle's engraving object as `bar`.

Two things LilyPond 2.24 was quietly refusing to draw:

`\bar ":|"` and the other old repeat names produce nothing at all — no
error, no warning, exit status 0, just a missing repeat that you find on
the tablet. Every book and forum answer still uses them, so translate
them to the modern spellings.

`\clef treble_8` unquoted is not an octavated clef either. It parses as a
plain treble plus a stray "8" markup that lands under the first note, and
the staff then reads an octave off — a tenor line engraved at soprano
pitch. Quote it.

The source pane, which is where either of those would have been visible,
is now a collapsed section at the bottom rather than a permanent slab. It
uses the panel's own disclosure helper, lifted out of Editor so both can
call it.

docs/bundle-format.md

@@ -174,6 +174,7 @@ it came from, in an `engraving` object.
     "key": "aes",
     "time": "4/4",
     "print_time": false,
+    "bar": 33,
     "voices": [
       { "clef": "treble",   "notes": "c4 des ees f | ees2. r4", "lyrics": "Kai -- paa -- va sy -- dän" },
       { "clef": "treble_8", "notes": "aes,4 aes aes aes | aes2. r4" },
@@ -190,6 +191,7 @@ it came from, in an `engraving` object.
 | `key` | string | optional | Key signature, in `lang`'s spelling. For `lilypond`, the tonic of the major spelling: `"aes"`, `"c"`, `"fis"`. |
 | `time` | string | optional | Time signature, as `"4/4"`. |
 | `print_time` | boolean | optional | Whether the time signature is printed on this system. Default `false`. |
+| `bar` | integer | optional | The measure this system starts at, as printed above its first bar. Omitted when the system is not numbered. |
 
 Each entry of `voices`: