clutter/actor: Disconnect from "layout-changed" signal on destroy

While the layout manager of a ClutterActor does get properly unset when
destroying an actor, we currently forget to disconnect the
"layout-changed" signal from it.

This can cause crashes when layout-changed is emitted after destroying
the actor.

GNOME/mutter!1281
6 jobs for !1281 with clutter-disconnect-layout-manager in 8 minutes and 57 seconds
detached
Status Job ID Name Coverage
  Review
passed #747828
check-commit-log

00:00:30

 
  Build
passed #747829
build-mutter

00:03:00

passed #747831
build-without-native-backend-and-wayland

00:02:35

passed #747830
build-without-opengl-and-glx

00:02:42

 
  Test
passed #747833
can-build-gnome-shell

00:01:44

passed #747832
test-mutter

00:05:26