Hypergrid renderer fixes

[texodus]

Fixes for @finos/perspective-viewer-hypergrid, which in sum greatly improve the user experience of using this plugin in the presence of slow interaciton with the core engine (e.g., when the engine is running on a remote server rather than a WebWorker).

• Fixed the plugin not appearing when the plugin attribute is toggled via setAttribute().
• Fixed the viewer flashing when a hover event is triggered before the page can be loaded, e.g. due to large or frequent updates.
• Fixed various timing-related console.error()s.

[codecov-io]

Codecov Report

Merging #764 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #764   +/-   ##
=======================================
  Coverage   92.41%   92.41%           
=======================================
  Files          47       47           
  Lines        3151     3151           
=======================================
  Hits         2912     2912           
  Misses        239      239

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5c73eb2...0cce313. Read the comment docs.