[Inspector UXR] Buttons and icons for debug toggles

_This issue has been broken out of https://github.com/flutter/devtools/issues/2890_

## Buttons and icons for debug toggles

Our developers didn’t understand what some of the debug modes did, or what their use cases are.


### Suggestions

* Provide clearer access to documentation about what problem each debug toggle is useful for solving. This could be achieved via explanatory tooltips with links to further documentation.
* Update iconography to be consistent (style and use of color).
* Consistent use of margins and paddings everywhere (buttons, icons, list, etc).
* Ensure that the buttons have tooltips when they’re shrunken down into an icon. Currently some of the buttons are missing their tooltips when shrunken down in size.
* Increase the contrast on selected view mode toggle buttons (repaint rainbow etc).
* The grouping of the toggle buttons feels pretty random. The context of the toggle buttons is too obscure at the moment. We could easily improve this by labelling the groups for example.

首先检查 Inspector 的调试切换控件和附加的屏幕截图，然后将列出的建议与当前 UI 进行比较。完成的标准是：这些切换控件能更清晰地访问文档，图标和间距保持一致，在紧凑模式下显示工具提示，选中状态具有更强的对比度，并且分组标签易于理解。

由索引模型根据 Issue 内容生成。