This removes the need to manually include the dialog builder in the main window's build() function.
It also removes the need to track if a dialog is visible.

Other changes:
- I made dialog.dart a bit more readable.
- I renamed transitionFinished to fullyOpaque since that's what actually matters.
- I made Routes track if they're opaque. Eventually this should probably be more configurable when creating the route.

Directions for Future Research:
- Use this for focus management somehow.
- The popup menu should use something like this.
- We should factor the following out into a showDialog() function that returns a future for the dialog's exit result:
    navigator.push(new DialogRoute(builder: (navigator, route) { ... }));
- Maybe navigator.pop() should take a value to return to that Future.