Speed up the rendering performance

[budnix]

Context

The PR includes fixes:

• Optimize the table selectors responsible for border-radius (thanks, @adrianspdev, for help);
• Fix missing htColumnHeaders class for nested headers;
• Optimize the selectors responsible for selection header borders (thanks, @adrianspdev, for help);

All those optimizations result in rendering the table almost twice as fast.

Navigation by arrow down from the topmost viewport:

Faster ~90% in fast render and 100% in slow render phase.

Navigation by arrow right from the middle of the viewport:

Faster ~70% in fast render and 40% in slow render phase.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Related issue(s):

1. https://github.com/handsontable/dev-handsontable/issues/2234

Affected project(s):

• handsontable

Checklist:

• I have reviewed the guidelines about Contributing to Handsontable and I confirm that my code follows the code style of this project.
• I have signed the Contributor License Agreement

[codesandbox-ci]

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Latest deployment of this branch, based on commit f655319:

Sandbox	Source
handsontable-pr-javascript-demo	Configuration
handsontable-ts-demo	Configuration
handsontable-pr-react-demo	Configuration

[github-actions]

Launch the local version of documentation by running:

npm run docs:review f655319fd16f670e546a4176c71765c3292a1003