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