Wayland only: window buttons can't be manually arranged

In a Wayland session from git xfce4-panel-4.19.4-85-g519b7568, you cannot properly rearrange window buttons manually.

Any button you drag results in the first and second window buttons being swapped. This is caused by the code here: https://gitlab.xfce.org/xfce/xfce4-panel/-/blob/519b7568aa0da228c9310af2d609060be199928e/plugins/tasklist/tasklist-widget.c#L37 which assigns all Wayland windows an id of 0, so the logic breaks down and presumes that the first and second window buttons satisfy the loop condition here: https://gitlab.xfce.org/xfce/xfce4-panel/-/blob/519b7568aa0da228c9310af2d609060be199928e/plugins/tasklist/tasklist-widget.c#L3618