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/gesture.ts

@@ -1,5 +1,5 @@
 import { Pointer } from "./types";
-import { normalizeScroll } from "./scene/data";
+import { normalizeScroll } from "./scene";
 
 export function getCenter(pointers: readonly Pointer[]) {
   return {