Show UI widget diagrams during code completion

[lukechurch]

As discussed with @danrubel

It would help during widget discovery if the nice graphics from here: https://flutter.io/widgets/ could be placed by the side of their classed during code completion. As a very rough mock:

Similar renderings could also appear in tooltips for the widgets

[mit-mit]

Would't the opposite work better? I mean, rather than having to type a widget type and seeing a preview of the result, what if you could pick from a visual list of widgets and have the corresponding code added?