Require a Key on Input.

Simplify the API for Focus.at() and Focus.moveTo().
Fixes #236.
This will require an e-mail to flutter-dev.

Make Input grab focus onTap not onPointerDown.
Fixes #189.

Complain when you use Focus.at() with two different GlobalKeys that
are both in the tree at the same time.
Fixes #181.

Add dartdocs for Focus.moveTo() and Focus.moveScopeTo().