Expose restacking and a window's stack layer to allow a compositor
to insert elements into the window stack in the right location.
(See Bug 571827 – hide panel when screensaver is active)
src/core/stack.h src/include/common.h: Move MetaStackLayer to
a public header.
src/core/screen.c src/core/screen-private.h src/core/stack.c:
Add a ::restacked signal emitted after we finish restracking.
src/core/window.h src/include/window.h: Add meta_window_get_layer()