Extract app and keys (#276)

* Extract app component from entrypoint (index)

- Use refs to refer to canvas and rough context
- Remove ReactDOM double rendering

* Extract keys and key related utils into their own module

* Move everything back to entrypoint

src/keys.ts

@@ -0,0 +1,23 @@
+export const KEYS = {
+  ARROW_LEFT: "ArrowLeft",
+  ARROW_RIGHT: "ArrowRight",
+  ARROW_DOWN: "ArrowDown",
+  ARROW_UP: "ArrowUp",
+  ENTER: "Enter",
+  ESCAPE: "Escape",
+  DELETE: "Delete",
+  BACKSPACE: "Backspace"
+};
+
+export const META_KEY = /Mac|iPod|iPhone|iPad/.test(window.navigator.platform)
+  ? "metaKey"
+  : "ctrlKey";
+
+export function isArrowKey(keyCode: string) {
+  return (
+    keyCode === KEYS.ARROW_LEFT ||
+    keyCode === KEYS.ARROW_RIGHT ||
+    keyCode === KEYS.ARROW_DOWN ||
+    keyCode === KEYS.ARROW_UP
+  );
+}