workspace: Remove skip-taskbar windows while the overview is shown

On startup desktop-icons-ng Wayland windows have skip-taskbar==FALSE so initially pass _isOverviewWindow and they get added to the overview, which is confusing to users. However almost immediately after that they get meta_wayland_client_hide_from_window_list and are removed from future overviews. So now we respond to notify::skip-taskbar immediately and prevent desktop-icons-ng appearing in the startup overview too.

This is messy and ideally we'd like to know the window type immediately on creation, but that option only exists in X11 and not Wayland.

https://gitlab.com/rastersoft/desktop-icons-ng/-/issues/137
https://bugs.launchpad.net/bugs/1936643