Optimize opening dropdown

[kazcw]

Pull Request Description

Text rendering accounts for most of the time spent opening a dropdown. The dropdown widget has been rendering its text twice: Once for the selected (bold) state of the text, and once for unselected. Deferring rendering of whichever version of each label is initially hidden halves the time for a dropdown to open.

Remove this eager rendering has the consequence that, when clicking on an item in a dropdown, 1-2 lines must be rendered to show the new selection state. I have not found the cost of this partial-redraw noticeable in my testing; it completes before we receive and react to the engine message that results from the graph change.

Before: 537.5ms to open the 6-item Join_Kind dropdown in Orders

After: 273.4ms to open the dropdown (50% improvement)

Important Notes

• Includes profiling annotations found useful in Investigate frontend performance #5603 / Optimize text rendering #5669.
• Updates the flamegraph renderer to use the same load-profile interface as the run-graph renderer.
• Removes some dead code.

Checklist

Please include the following checklist in your PR:

• The documentation has been updated if necessary.
• All code conforms to the
  Scala,
  Java,
  and
  Rust
  style guides.
• All code has been tested:
  • Unit tests have been written where possible.
  • If GUI codebase was changed: Enso GUI was tested when built using BOTH
    ./run ide build and ./run ide watch.

[wdanilo]

That's amazing!