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/man/weston.ini.man b/man/weston.ini.man
index 22f9f0d..fbcec81 100644
--- a/man/weston.ini.man
+++ b/man/weston.ini.man
@@ -201,6 +201,12 @@
 .B none.
 By default, no animation is used.
 .TP 7
+.BI "close-animation=" fade
+sets the effect used when closing windows (string). Can be
+.B fade,
+.B none.
+By default, the fade animation is used.
+.TP 7
 .BI "startup-animation=" fade
 sets the effect used for opening new windows (string). Can be
 .B fade,