Refactor (#862)

* Initial factoring out of parts of the LayerUI component

2360 → 2224 LOC

* Create a Section component

* Break up src/index.tsx

* Refactor actions to reduce duplication, fix CSS

Also consolidate icons

* Move scene/data.ts to its own directory

* Fix accidental reverts, banish further single-character variables

* ACTIVE_ELEM_COLOR → ACTIVE_ELEMENT_COLOR

* Further refactoring the icons file

* Log all errors

* Pointer Event polyfill to make the tests work

* add test hooks & fix tests

Co-authored-by: dwelle <luzar.david@gmail.com>

src/components/Section.tsx

@@ -0,0 +1,27 @@
+import React from "react";
+import { t } from "../i18n";
+
+interface SectionProps extends React.HTMLProps<HTMLElement> {
+  heading: string;
+  children: React.ReactNode | ((header: React.ReactNode) => React.ReactNode);
+}
+
+export function Section({ heading, children, ...props }: SectionProps) {
+  const header = (
+    <h2 className="visually-hidden" id={`${heading}-title`}>
+      {t(`headings.${heading}`)}
+    </h2>
+  );
+  return (
+    <section {...props} aria-labelledby={`${heading}-title`}>
+      {typeof children === "function" ? (
+        children(header)
+      ) : (
+        <>
+          {header}
+          {children}
+        </>
+      )}
+    </section>
+  );
+}