wayland: Delay processing of xdg-decoration::destroy() requests
The xdg_toplevel_decoration must be destroyed before the xdg_toplevel surface. However, if we process the destructor request immediately and the window is about to be closed, the window will be frameless while it's being animated, which doesn't look eyecandy.
With this change, kwin is going to delay processing of the destroyed() signal for the xdg_decoration in case the xdg_toplevel surface is about be destroyed.