Improve treemap navigation

[kenzieschmoll]

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