wayland: Update window caption suffix on map

Window captions have a suffix for disambiguation, based on window
captions of other mapped windows.

In xdgshell we update this when a caption changes, but this could be
before a window is mapped. This can lead to two windows at once having
the same suffix, as a suffix could be used but not in the list of
searched clients.

BUG: 450369

src/waylandwindow.cpp

@@ -291,6 +291,7 @@ void WaylandWindow::markAsMapped()
 {
     if (Q_UNLIKELY(!ready_for_painting)) {
         setupCompositing();
+        updateCaption();
         setReadyForPainting();
     }
 }