actor: Provide add/remove child and get children methods

actor: Provide add/remove child and get children methods

Let's try and move away from the reverse implicit scene graph build API,
which we mutuated from GTK+, towards a more traditional node/child API.

The set_parent()/unparent() API is confusing, unless you know the
history; having a add_child()/remove_child() methods pair makes it more
explicit.

We can easily implement the old set_parent()/unparent() pair in terms of
the newly add_child()/remove_child() one.