Add Wayland support
!38 (merged) fixed the crash at startup on Wayland (#73 (closed)), but it remains to add a real Wayland support as much as possible: replace libwnck by libxfce4windowing (even if it is a partly virtual support for now in XfwApplication), adapt the build, etc., as it was done for xfdesktop (xfce/xfdesktop!43 (merged)) or xfce4-panel (xfce/xfce4-panel!103 (merged)).