compositor: add destructors to shell plugins

Add a new wlsc_shell API function for destroying the shell plugin
object. Helps to reduce Valgrind reports.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
diff --git a/compositor/compositor.c b/compositor/compositor.c
index 0fa9ed8..06ee7f3 100644
--- a/compositor/compositor.c
+++ b/compositor/compositor.c
@@ -2172,6 +2172,8 @@
 	if (xserver)
 		wlsc_xserver_destroy(ec);
 
+	ec->shell->destroy(ec->shell);
+
 	if (ec->has_bind_display)
 		ec->unbind_display(ec->display, display);