Support message sending

[dhardy]

• Add fn EventState::send<M: Debug + 'static>(&mut self, id: Id, msg: M) to send a type-erased message to a designated target
• Add fn ConfigCx::push<M: Debug + 'static>(&mut self, id: Id, msg: M) to emit a type-erased message from configure/update

Currently the second method is just a wrapper around the first, but (a) there might be a distinction between "push/emit a reusable message" and "send a non-reusable message" in the future and (b) ConfigCx might gain an embedded Id for the current widget, in which case push would lose the id parameter (but send would not).

handle_messages impls: do not expect cx.last_child().is_some()

Since it's now possible to send a message, handle_messages cannot assume that a message comes from a child.