Add parent & toplevel functions

These are intended to make it easier to navigate "up" the hierarchy of objects.

Example usage:
```
destroy(toplevel(imgc.c))
```
will close the window associated with a canvas in `imgc.c`. Triggered by https://github.com/timholy/ImageView.jl/issues/3

@jverzani, please let me know if there's a better way to do this, otherwise either of us can merge.