Remove _sort_children from script bindings:
_sort_children is an internal method which shouldn't be exposed to scripts.

Added support for non-bound methods in MessageQueue:
So we can use deferred calls without exposing internal methods to scripts.

Added debug checks in CallableCustomMethodPointer:
Adding method pointer callables to the message queue was causing crashes
in case an object was destroyed and the same memory was allocated for
another one. The new object had a valid object id but the call was erroneous.
Release will be fixed later, along with Variant which has the same problem and
is also fixed for debug only.