Add an IContainer Interface

There is already an informal interface used in a number of places where a `Widget` which contains other widgets, such as `IDialog`, `IApplicationWindow`, and possibly others, expect subclasses to override the `_create_contents` method to populate and layout the main area of the control. These widgets may have additional contents beyond these things (eg. a dialog's buttons), so this method provides a hook for customization.

The interface would look something like:
```
class IContainer(IWidget):
def _create_contents(self, parent):
pass
```
where `parent` is the parent widget for the contents (usually, but not always, `self.control`) and the method is expected to return a toolkit object.

Such an interface would be useful for a hypothetical `IPanel` (a possibly named/framed group within a larger UI).

By formalizing this interface, new classes would be more likely to follow it.