Improve treemap navigation

We could do things like
- [ ] highlight the hovered cell on mouse over to make it look more clickable
- [ ] give the treemap blocks thicker borders to make the hierarchy more clear
- [x] make breadcrumbs more obviously clickable (done in https://github.com/flutter/devtools/pull/2303)
- [ ] add concept of back / forward (similar to debugger)
- [ ] override the chrome back button to go back to previously selected treemap node

未指定任何文件、测试或入口点。首先定位 treemap navigation UI 及其现有的 breadcrumb 行为，然后查看已链接的 pull request，以确认已完成的 breadcrumb item；当剩余的 hover、border、back/forward 和 Chrome back-button 行为按描述正常工作时，即表示完成。

由索引模型根据 Issue 内容生成。