Rework `Allocations` Tab in Memory Page

[bkonyi]

Feature work:

• [x] Reimplement allocation profile (e.g., breakdown memory + # of instances by class) (https://github.com/flutter/devtools/pull/4257)
• [x] Reimplement allocation tracing (e.g., record and display stack traces for allocations of specified classes)
  • [x] https://github.com/flutter/devtools/pull/4364

Cleanup:

• [x] Sort table by memory consumed rather than by class name
  • [x] In the tab Profile
  • [x] In the tab Tracing
• [ ] explain in documentation what is included in inner (shallow) size of an object
• [x] https://github.com/flutter/devtools/issues/4300
• [ ] Add button 'Deselect All' for tracing
• [x] https://github.com/flutter/devtools/issues/4484
• [ ] Support button 'Clear' and 'Export' on the control pane?

cc @polina-c

[polina-c]

Related issues: https://github.com/flutter/devtools/issues/4315 https://github.com/flutter/devtools/issues/4314

[InMatrix]

Is there a separate bug tracking documentation updates for the new memory screen? These two docs will become out of date once this change land in stable:

• https://docs.flutter.dev/development/tools/devtools/memory
• https://github.com/flutter/devtools/blob/master/case_study/memory_leaks/images_1_null_safe/README.md