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.

noteman_slicer/lilypond.py

@@ -12,6 +12,7 @@ scaling.
 
 from __future__ import annotations
 
+import re
 import shutil
 import subprocess
 import tempfile
@@ -63,6 +64,25 @@ RELATIVE_REFERENCE = {
     "bass": "c",
 }
 
+# LilyPond renamed the repeat barlines and silently draws *nothing* for the old
+# names — no error, no warning, just a missing repeat that you find on the
+# tablet. Every book, every forum answer and every score anyone has typed before
+# uses the old ones, so translate them.
+_BAR_ALIASES = {
+    "|:": ".|:",
+    ":|": ":|.",
+    ":|:": ":|.|:",
+    "||:": ".|:",
+    ":||": ":|.",
+    ":||:": ":|.|:",
+}
+_BAR = re.compile(r'(\\bar\s*")([^"]*)(")')
+
+
+def _modernise_bars(notes: str) -> str:
+    return _BAR.sub(lambda m: m[1] + _BAR_ALIASES.get(m[2], m[2]) + m[3], notes)
+
+
 _PREAMBLE = """\\version "2.24.0"
 \\paper {
   indent = 0\\mm
@@ -85,17 +105,35 @@ def generate(replacement, key: str, time: str) -> str:
     key = replacement.key or key
     time = replacement.time or time
 
+    # Bar numbering is a Score property, so it is set once, on the first staff.
+    # Visible at the beginning of a line and nowhere else — which in a
+    # one-system slice means exactly one number, above the first bar, the way a
+    # printed score numbers its systems. The empty bar line is what gives the
+    # number a line beginning to attach to.
+    number = ""
+    if replacement.bar:
+        number = (
+            f"      \\set Score.currentBarNumber = #{int(replacement.bar)}\n"
+            "      \\override Score.BarNumber.break-visibility = #'#(#f #f #t)\n"
+            '      \\bar ""\n'
+        )
+
     staves = []
     for voice in replacement.voices:
         hide = "" if replacement.print_time else "      \\omit Staff.TimeSignature\n"
-        body = voice.notes.strip() or "s1"
+        body = _modernise_bars(voice.notes.strip()) or "s1"
         reference = RELATIVE_REFERENCE.get(voice.clef, "c'")
         staff = (
             "    \\new Staff {\n"
             f"{hide}"
-            f"      \\clef {voice.clef}\n"
+            # Quoted, because an octavated name has to be: unquoted,
+            # `\clef treble_8` parses as a plain treble clef with a stray "8"
+            # markup that lands under the first note, and the staff then reads
+            # an octave off.
+            f'      \\clef "{voice.clef}"\n'
             f"      \\key {key} \\major\n"
             f"      \\time {time}\n"
+            f"{number if not staves else ''}"
             f"      \\relative {reference} {{ {body} }}\n"
             "    }\n"
         )