The old names were getting silly and started stepping on valuable namespace.

The new names are consistent and clear.

Previously the widgets layer only provided a concrete binding, which
makes it awkward to extend it compared to other bindings. This moves
widgets to the same style as the other layers.

In a subsequent patch I'll use this to make the tests layer saner.

Adds BuildOwner to manage the dirty list and build processing for
widgets/elements, and adds a widget unit test to make sure separation
is enforced.

Fixes #2723

Fixes #2723

And rootConstraints to configuration.

This type and its variable have nothing to do with Constraints.

Fixes https://github.com/flutter/flutter/issues/829