Stub 'clear()' method to allow System to clear circular references.

SystemModule.pm

@@ -64,6 +64,19 @@ sub new {
 }
 
 
+# ============================================================================
+#  Clean shutdown support
+
+## @method void clear()
+# A function callable by System to ensure that any circular references do not
+# prevent object destruction.
+sub clear {
+    my $self = shift;
+
+    # The method, it does nothing!
+}
+
+
 # ============================================================================
 #  Error functions