Provide UI affordances for visually editing widget constructor parameters

[InMatrix]

Problem

Widget constructor parameters and their values are sometimes difficult to discover and make sense of without diving into the widget's documentation. In user studies, we've seen participants misspell parameter names, forget to set a critical parameter's value, or use the wrong parameter for the intended effect.

In addition, some parameter values are not intuitive without a visual aid, e.g., BoxFit.

Proposal

Extend #4028 to include more parameters such as those use Curves, BoxFit, MainAxisAlignment, or other enums. We can potentially reuse existing diagrams for those enums in their API docs.