Introduce layer window rule & drop window type rule
The new window rule allows to overwrite the stack layer. It can be useful on wayland to force picture-in-picture surfaces (which are xdg-toplevels at the moment) to be placed above fullscreen windows. Keep above flag is unsuitable because fullscreen windows are placed higher "above" windows.
Practically all code assumes that the window type is static and fixing it would bring extra complexity, which may not be worth given that there are window rules to control position, focus, layer, etc.
closes #199 (closed)
Edited by Vlad Zahorodnii