window-list: Fix icons not showing for some windows on wayland
On wayland, the properties that are used for application matching are generally set after the window has been created, so it is normal that buttons start with the fallback icon. While we already track the properties that are relevant for app matching, our signal handler may run before the window is matched to its app. Make sure the WindowTracker gets to process those signals first by using connect_after() for our own handlers. Fixes #10.