Workspace/WorkspaceView changes

This is a potential API for atom.workspace. The api's for Workspace and WorkspaceView are combined to create a single more approachable API.
I think we should refer to these objects as "panels". The contract for attaching them should be the same as for pane items, and I think we should support the following options:

* A React component.
* A plain JS object with a `::getDOMNode()` method and some lifecycle hooks that we will call. That allows people to write adaptors to whatever framework they want.

Less recommended path for flexibility backward compatibility:
* A SpacePen view / jQuery wrapper
* A raw DOM element.