Layer::Window overlays: an OverlaySpec can now be an ordinary xdg toplevel, named through App::overlay_title; showcase persists the last pressed button too

A shell built on ltk had no way to show something as a regular window: every overlay is a layer-shell surface, and layer-shell surfaces are either below all application windows or above all of them. crustace's "closing applications…" card ran into exactly that — on the overlay layer it covered the applications' own "save changes?" prompts, on the bottom layer it vanished behind their windows — and what that card wants is to be one window among the others: decorated by the compositor, stackable, listed in the switcher and the dock.
`Layer` gains a `Window` variant. An `OverlaySpec` carrying it is materialised by the reconciler as an `xdg_toplevel` (sctk `Window`, server-side decorations) instead of a layer surface: `size` is its fixed size, applied through min/max size (a zero component falls back to a default, since a toplevel cannot "fill"); anchor, exclusive zone and keyboard exclusivity are ignored. The window's app_id is `App::app_id()` and its title comes from the new, defaulted `App::overlay_title( id )`, so the compositor's window lists show something meaningful. `WindowHandler::configure` and `request_close` now resolve which surface a window belongs to: the main window keeps its behaviour, an overlay window is configured through its own `SurfaceState` (with the window geometry set to the configured size) and the compositor's close button delivers the spec's `on_dismiss` instead of exiting the app. Layer-shell applications now bind `xdg_wm_base` when the compositor offers it, so they can open such a window later. Adding a variant keeps every existing `OverlaySpec` literal compiling; the new trait method has a default.
The showcase example now also saves and restores `last_pressed`, bumping its private state format to version 2 with the note kept last so its line breaks survive.

src/event_loop/run.rs

@@ -218,6 +218,9 @@ pub( crate ) fn try_run<A: App>( mut app: A ) -> Result<(), RunError>
 			}
 		}
 	};
+	// Layer-shell apps still get xdg_wm_base when the compositor has it, so
+	// a `Layer::Window` overlay can be created later.
+	let xdg_shell = xdg_shell.or_else( || XdgShell::bind( &globals, &qh ).ok() );
 
 	// Bind the session-lock manager and request the lock. We don't keep the
 	// `SessionLockState` (the manager) around: it has no `Drop`, so the