ui: Add baseline diffing to the tree explorer; use it in HDE

Make flamegraph diffing an opt-in feature of the tree explorer panel,
next to its Call Tree and Functions table views, and make the Heap
Dump Explorer its first user.

Tree explorer:
- TreeExplorerPanelAttrs.baseline opts into comparing the panel's tree
  against a baseline tree, fetched with the same state (metric,
  filters, view). Hosts without a baseline are unaffected. For now,
  only trees of the same trace are supported.
- buildTreeExplorerDiff pairs the nodes of the two trees by name and
  grouping properties under paired parents, pprof-style, and lays out
  their union as one tree. Columns marked profileSpecific (e.g. hashes
  of ids) are ignored when pairing.
- state.comparison chooses what is shown (the diff, or either tree on
  its own), what diff node widths are proportional to (both trees, or
  one of them) and what node colors encode (absolute or relative
  change).
- A Diff switch and a Baseline/Current toggle at the right of the tab
  bar choose between the diff and either tree. A legend and a tune
  menu control how the diff is drawn.
- The flamegraph colors diff nodes from green (shrank) to red (grew).
  Its tooltips, copied stacks and exports show the baseline and current
  values and their change, and only the properties which pair nodes, as
  the others have a value per tree. Host node actions are not offered
  on nodes only the baseline has.
- The Call Tree and Functions tabs show the diff with baseline, change
  and relative change columns, largest growth first.

Heap Dump Explorer:
- A "Compare with:" picker on the Flamegraph tab selects a baseline
  dump of the same trace. It is persisted with the rest of the state.
- Heap dump labels include the dump's time, to tell apart the dumps of
  one process.
- The pprof download holds the dump being shown.

Merging several profiles is not part of this change.