Add `clipBehavior` to `DialogTheme`

### Use case

Right now, we can specify the `Clip clipBehavior` of dialog when building them:


```dart
Dialog(
  clipBehavior: Clip.hardEdge,
);
```

It would be nice to be able to specify it in the theme so it applies to all dialogs in the app.

### Proposal

Add a new property `Clip? clipBehavior` to `DialogTheme` that would be used by the dialogs.