feat(apprt-gtk): port the GTK4/libadwaita app runtime

Ports ZIG/src/apprt/gtk into ghostty-apprt-gtk: every GObject class
(application, window, tab, split tree, surface + scrolled window,
child-exited banner, search/key-state/resize overlays, command palette,
global shortcuts, all dialogs, config wrapper) as glib subclasses with
the same GType names, properties, signals and template children; GDK
key translation; X11 and Wayland window-protocol helpers (blur, CSD,
quick terminal via gtk4-layer-shell); single-instance D-Bus IPC;
gsettings; cgroup scope creation.

build.rs compiles Ghostty's blueprints with blueprint-compiler and
bundles UI, CSS and icons into a gresource, as Ghostty's build does.

The core is not ported yet, so the crate defines the boundary it needs
in src/core.rs (CoreApp, CoreSurface, RtApp, RtSurface, Waker); the
spec is in docs/rewrite/notes/ghostty-apprt-gtk.md. ghostty-core will
implement these traits (the crate no longer depends on core to avoid a
cycle). examples/demo.rs runs the app with a dummy core that clears the
GLArea; it launches on Wayland without GTK criticals.

Stubs/open items: toggle_visibility, float_window, secure_input and
close_all_windows return false; inspector is a no-op; X11/Wayland
protocol code is untested against real compositors; gtk4-rs doesn't
expose the GLArea framebuffer id, so the renderer must query it.

Also registers ghostty-apprt and ghostty-cli in workspace.dependencies.

Test Plan:
- cargo test -p ghostty-apprt-gtk (36 tests)
- DEMO_QUIT_AFTER_MS=3000 cargo run -p ghostty-apprt-gtk --example demo

crates/ghostty-apprt-gtk/src/ipc/new_window.rs

@@ -0,0 +1,43 @@
+//! `+new-window` IPC (port of `apprt/gtk/ipc/new_window.zig`).
+//!
+//! Use a D-Bus method call to open a new window on GTK. See
+//! <https://wiki.gnome.org/Projects/GLib/GApplication/DBusAPI>.
+//!
+//! `ghostty +new-window` is equivalent to (release build):
+//!
+//! ```text
+//! gdbus call --session --dest com.mitchellh.ghostty --object-path /com/mitchellh/ghostty --method org.gtk.Actions.Activate new-window [] []
+//! ```
+//!
+//! `ghostty +new-window -e echo hello` is equivalent to:
+//!
+//! ```text
+//! gdbus call --session --dest com.mitchellh.ghostty --object-path /com/mitchellh/ghostty --method org.gtk.Actions.Activate new-window-command '[<@as ["-e" "echo" "hello"]>]' []
+//! ```
+
+use ghostty_apprt::ipc::{Error, NewWindow, Target};
+use gtk::prelude::*;
+
+use super::dbus::DBus;
+
+pub fn new_window(target: &Target, value: &NewWindow) -> Result<bool, Error> {
+    let mut dbus = DBus::new(
+        target,
+        if value.arguments.is_none() {
+            "new-window"
+        } else {
+            "new-window-command"
+        },
+    )?;
+
+    if let Some(arguments) = &value.arguments {
+        // If any arguments were specified on the command line, the first
+        // parameter is an array of strings that contain the arguments. They
+        // will be sent to the main Ghostty instance and interpreted as CLI
+        // arguments.
+        dbus.add_parameter(&arguments.to_variant());
+    }
+
+    dbus.send()?;
+    Ok(true)
+}