Port some more tests to fn3

Add fling to Scrollable.

Port some more tests to fn3

Port Input and EditableText to fn3

Port even more examples to fn3

Remove the "update" notification logic.
Hook the register/unregister functions. We're still not notifying.

ComponentState becomes State, for brevity.
Instead of overriding its constructor, override initState().
This makes writing States much simpler.

Port Date Picker example to fn3

Also:
- Expose the slot of each Element.
- Minor improvements to HomogeneousViewport.
- Replace TestComponent with FlipComponent in tests.

This changes how PopupMenu works slightly because there's no long an onPressed
callback on PopupMenuItem. Instead, callers should use showPopupMenu.

Updates for change in Mojom dart enum generation

Port most of scrollable.dart to fn3

Port some drawer and button widgets to fn3

Update to Mojo 4e4d51ce28a8edcb32b9c7f555e38e2ae84a825e, update deps

Fix the minedigger game.

This updates to mojo 4e4d51ce28a and mojo sdk 711a0bcfb141b4 and updates the sky
package's pubspec.yaml dependency to '>=0.1.0 <0.2.0' to be compatible with
the current mojo package. This includes an update to the Mojo Dart generator to
produce real classes for enums and the corresponding updates for users of the
KeyboardType enum in Sky as well as one scoped_ptr->std::unique_ptr in shell
corresponding to a change in the Mojo EDK.

When a new version of the sky and sky_services package are pushed this will fix
domokit/mojo#440.

Remove the Widgets framework's PointerEventListener definition now that
the Rendering layer has one. It was previously clashing in files that
imported both, which would result in it being hidden. Turns out
MineDigger is the only example that this affects.

This change ensures that we only store the results of a hit
test on the initial pointer down event.  Moreover, we perform
new hit tests each time a hovering pointer moves.  This is
important to ensure correct behavior of input devices which can
hover, such as mice.

Previously the first hover movement after releasing a mouse
button would cause a new pointer state to be recorded along with
hit test results for wherever the pointer happened to be which
caused the following pointer down event to be delivered to the
wrong place.

Fixes issue #1189.