Data Module: Expose state using discoverable selectors

[atimmer]

This continues the work of #4105 by making selectors discoverable. All selectors are available on the wp.data.selectors object. The data module handles passing the state to the selectors. So a consumer of wp.data.selectors can use these functions without knowing about the state.

Using the HoC looks like this:

const MyComponentWithData = query( ( selectors ) => {
	return {
		title: selectors[ 'core/editor' ].getEditedPostTitle(),
	};
} )( props =>
  <div>{ props.title || 'no title' }</div>
);

cc @youknowriad, @gziolo, @mcsf, @moorscode

Closes #4105
Closes #4083
Closes #2473

[atimmer]

Superseded by #4105