Implement support for XDG window decoration

* 12to11.c (XLMain): Initialize window decoration.
* Imakefile (SRCS, OBJS): Add decoration.c and decoration.o.
* README: Document support for zdg_decoration_manager_v1.
* compositor.h: Update prototypes.
* xdg_toplevel.c (XdgDecoration, DecorationMode): New types.
(enum _DecorationMode): New structures.
(struct _XdgToplevel): New fields `decor' and `decoration'.
(struct _XdgDecoration): New struct.
(SendDecorationConfigure): New function.
(Commit): Apply decoration should it be dirty.
(HandleResourceDestroy): Detach decoration object.
(Destroy): Post error if decoration object would be orphaned.
(DestroyDecoration, SetMode, UnsetMode)
(HandleDecorationResourceDestroy, XLXdgToplevelGetDecoration):
New functions.

Imakefile

@@ -21,7 +21,7 @@ SRCS = 12to11.c run.c alloc.c fns.c output.c compositor.c       \
   dmabuf.c buffer.c select.c xdata.c xsettings.c dnd.c          \
   icon_surface.c primary_selection.c renderer.c                 \
   picture_renderer.c explicit_synchronization.c transform.c     \
-  wp_viewporter.c
+  wp_viewporter.c decoration.c
 
 OBJS = 12to11.o run.o alloc.o fns.o output.o compositor.o       \
   surface.o region.o shm.o atoms.o subcompositor.o positioner.o \
@@ -30,7 +30,7 @@ OBJS = 12to11.o run.o alloc.o fns.o output.o compositor.o       \
   dmabuf.o buffer.o select.o xdata.o xsettings.o dnd.o          \
   icon_surface.o primary_selection.o renderer.o                 \
   picture_renderer.o explicit_synchronization.o transform.o     \
-  wp_viewporter.o
+  wp_viewporter.o decoration.o
 
 GENHEADERS = transfer_atoms.h
 
@@ -110,6 +110,7 @@ ScannerTarget(xdg-shell)
 ScannerTarget(primary-selection-unstable-v1)
 ScannerTarget(linux-explicit-synchronization-unstable-v1)
 ScannerTarget(viewporter)
+ScannerTarget(xdg-decoration-unstable-v1)
 
 /* Make OBJS depend on scanner headers, and depend on both them and SRCS.  */
 $(OBJS): $(GENHEADERS)