[Property Editor] Show widget name and documentation in property list

[elliette]

It would be useful to show the widget's name above the list of properties in the property editor.

This requires including the name of the widget in the response sent back from the Analysis Server, e.g.:

{
   "name": "Center", // <-- widget name
   "documentation": "Creates a widget that centers its child.", // <-- documentation
    "arguments": [
      {
        "name": "widthFactor",
        "type": "double",
        "value": 10.0,
        "isDefault": false,
      },
     ...

The documentation corresponds to the short description shown when hovering on the widget name in a code editor:

Note: I've also added this to the Property Editor API spec.