Make bindings unloadable

- IPC methods are bound in Binding.bind_methods(), not in constructor.
- Add method Binding.remove() as reverse action to Binding.add()
- Add method Bindings.remove_object() as reverse action to
  Bindings.add_object()
- Add methods object_added() and object_removed() to (dis)connect
  signals.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>