Introduce layer window 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.

CCBUG: 466016
(cherry picked from commit 5c4acbdd)