Add indication for lack of WidgetInspector

A WidgetInspector widget is needed to do widget inspection inside of an app. If there is none present then DevTools does not communicate that clearly.

It would be helpful to let the user know that no WidgetInspector was found, so they know they need to add one.

Furthermore it may be useful to let the user know which widgets are inside of the WidgetInspector, and therefor able to be inspected.

Issue 中没有指定文件或测试。首先定位 DevTools UI 以及对 WidgetInspector 的处理，然后确定没有 inspector 时是如何表示的，以及当前如何展示被检查的 widgets。完成的标准是 DevTools 能够明确报告没有可用的 WidgetInspector，并且如果现有 UI 支持，能够识别它可以检查的 widgets。

由索引模型根据 Issue 内容生成。