book: add "Don't keep locks while emitting signals" section

[mvanhorn]

Closes #392.

Summary

#392 asked for a small section in the signals chapter explaining why a
RefCell borrow (or a Mutex guard) must not be held across signal
emission, with a self-contained example that demonstrates the problem
and the fix. The maintainer's suggestion in the thread was to use a
String-typed property as the easiest way to surface the issue.

What's in the listing

book/listings/g_object_signals/3/:

• tracked_button/imp.rs - a custom button with a text: RefCell<String> property. ButtonImpl::clicked clones the String
  out of the RefCell first, drops the borrow, and only then calls
  emit_by_name::<()>("text-changed", &[&snapshot]). An
  ANCHOR: clicked_good block delimits the part the chapter pulls in
  via {{#rustdoc_include ...}}.
• tracked_button/mod.rs - the public wrapper, mirroring the listing-2
  shape exactly.
• main.rs - the handler intentionally calls back into the button by
  calling set_text from inside text-changed. This is the load-
  bearing detail: it only works because clicked dropped the borrow
  before emitting. If the borrow had survived, the inner set_text
  would have aborted at BorrowMutError. An ANCHOR: handler block
  delimits the embedded snippet for the chapter.

What's in the chapter

book/src/g_object_signals.md gets a "Don't keep locks while emitting
signals" section just before the closing summary. It states the rule
once, embeds the clicked_good snippet, embeds the handler, and
explains why the fact that the handler doesn't panic is the actual
demonstration of the rule. It also notes that property notifications go
through the same dispatch path, so the same rule applies to notify.

Wiring

book/listings/Cargo.toml gets a g_object_signals_3 [[bin]] entry
between the existing g_object_signals_2 and g_object_subclassing_1,
matching the existing format.

Verification

• The new files mirror listing 2's layout (mod.rs wrapping imp,
  imp.rs deriving Properties + glib::object_subclass + signals
  via OnceLock<Vec<Signal>>, glib::derived_properties on
  ObjectImpl).
• I could not run cargo check locally because GTK 4 system libraries
  (gtk4.pc, pkg-config) aren't available on this machine; I tracked
  the existing patterns line-for-line against listings/g_object_signals/2/
  and book/listings/Cargo.toml so the wiring matches what the rest of
  the book does.
• The chapter rendering uses the same {{#rustdoc_include ../listings/...}} mechanism the rest of the chapter already uses,
  with named anchors (clicked_good, handler) that match the
  existing object_impl/button_impl/signal_handling style.

If anything fails to compile under GTK 4, happy to amend - the example
is small enough to iterate quickly.

Out of scope

• I did not add a "broken on purpose" listing. The section explains the
  bad pattern in prose and shows the good pattern in code, which keeps
  the listing buildable as part of g_object_signals_3 and avoids
  cluttering the book's cargo check with an example that's expected
  to panic.
• No changes to the existing g_object_signals_1 / _2 listings.