[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.

Start by reviewing the Inspector debug-toggle controls and the attached screenshot, then compare the listed suggestions with the current UI. Done means the toggles have clearer documentation access, consistent icons and spacing, tooltips in compact mode, stronger selected-state contrast, and understandable grouping labels.

索引モデルが issue の本文から書いたものです。