gtk(wayland): explicitly set layer name

Even though gtk4-layer-shell's documentation claims that "nobody quite
knows what it's for", some compositors (like Niri) can define custom
rules based on the layer name and it's beneficial in those cases to
define a distinct name just for our quick terminals.

pkg/gtk4-layer-shell/src/main.zig

@@ -50,3 +50,7 @@ pub fn setMargin(window: *gtk.Window, edge: ShellEdge, margin_size: c_int) void
 pub fn setKeyboardMode(window: *gtk.Window, mode: KeyboardMode) void {
     c.gtk_layer_set_keyboard_mode(@ptrCast(window), @intFromEnum(mode));
 }
+
+pub fn setNamespace(window: *gtk.Window, name: [:0]const u8) void {
+    c.gtk_layer_set_namespace(@ptrCast(window), name.ptr);
+}