desktop-shell: add close-animation config option for destroying surfaces

At the moment when surfaces are destroyed they are faded out but let's
make it configurable!

Tested-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/desktop-shell/shell.h b/desktop-shell/shell.h
index 6e63785..e994cdb 100644
--- a/desktop-shell/shell.h
+++ b/desktop-shell/shell.h
@@ -194,6 +194,7 @@
 	uint32_t binding_modifier;
 	uint32_t exposay_modifier;
 	enum animation_type win_animation_type;
+	enum animation_type win_close_animation_type;
 	enum animation_type startup_animation_type;
 	enum animation_type focus_animation_type;