Initial implementation of `Tracing` tab

[bkonyi]

This is an initial implementation of an allocation tracing tab for the memory screen. Current functionality includes:

• Enabling / disabling tracing for individual classes
• Refreshing tracing data
• Bottom-up tree view of allocation locations for traced classes, with inclusive and exclusive counts similar to those found on CPU profiles
• Expand / Collapse all support

Functionality that still needs to be implemented includes:

• Multi-isolate support
• Top-down tree view support
• Class list search filtering
• Improved table behavior when enabling / disabling tracing for classes. In particular, inserting a duplicate element into the list of classes when enabling tracing instead of moving the existing element to the top of the list.
• Support for serialization/deserialization

Tracking issue: https://github.com/flutter/devtools/issues/4335

Tracing not enabled:

Tracing enabled with a selected allocation profile:

[bkonyi]

I still need to write tests, but figured this is worth an initial review pass.

[bkonyi]

can you move packages/devtools_app/test/test_data/allocation_trace.json to test_data/memory/allocation_tracing/?

Done.