Dismiss presented controller when parent controller goes away

[stephencelis]

This brings parity with SwiftUI, where if a view disappears, an attached sheet disappears shortly thereafter.

[stephencelis]

This closely matches SwiftUI's behavior, which is quite glitchy: when the view disappears, the sheet stays alive till the next render frame, and then is removed immediately without animation. I think technically our behavior is slightly less glitchy in that the render tick seems shorter, but it's still not great behavior, it just avoids an inconsistent view/model state.

• I'd like to write a test for this before merging
• I'd like to make sure this plays nicely with controllers that are pushed and not merely presented