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.

src/apprt/gtk/winproto/wayland.zig

@@ -110,6 +110,7 @@ pub const App = struct {
 
         layer_shell.initForWindow(window);
         layer_shell.setLayer(window, .top);
+        layer_shell.setNamespace(window, "ghostty-quick-terminal");
     }
 
     fn registryListener(