Improve treemap navigation #2314

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
• make breadcrumbs more obviously clickable (done in Use new BreadcrumbNavigator widget in code size screen #2303)
• add concept of back / forward (similar to debugger)
• override the chrome back button to go back to previously selected treemap node