feat: rewrite public UI component rendering using tunnels (#6117)

* feat: rewrite public UI component rendering using tunnels

* factor out into components

* comments

* fix variable naming

* fix not hiding welcomeScreen

* factor out AppFooter and memoize components

* remove `UIOptions.welcomeScreen` and render only from host app

* factor out tunnels into own file

* update changelog. Keep `UIOptions.welcomeScreen` as deprecated

* update changelog

* lint

---------

Co-authored-by: Aakansha Doshi <aakansha1216@gmail.com>

src/excalidraw-app/components/AppFooter.tsx

@@ -0,0 +1,21 @@
+import React from "react";
+import { Footer } from "../../packages/excalidraw/index";
+import { EncryptedIcon } from "./EncryptedIcon";
+import { ExcalidrawPlusAppLink } from "./ExcalidrawPlusAppLink";
+
+export const AppFooter = React.memo(() => {
+  return (
+    <Footer>
+      <div
+        style={{
+          display: "flex",
+          gap: ".5rem",
+          alignItems: "center",
+        }}
+      >
+        <ExcalidrawPlusAppLink />
+        <EncryptedIcon />
+      </div>
+    </Footer>
+  );
+});