Reunite split sidebar, add ability to register action buttons to it (#3100)

* Rework ribbon to support panes and actions

* Restore nice focus-within highlighting

* A better export icon

* Fix up some issues with tests due to sidebar and tooltip tweaks

* A snapshot a day keeps the bugs away! 📷🐛 (OS: ubuntu)

* Re-run CI

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

src/components/CustomIcon.tsx

@@ -223,6 +223,16 @@ const CustomIconMap = {
       />
     </svg>
   ),
+  floppyDiskArrow: (
+    <svg viewBox="0 0 20 20" fill="none" xmlns="http://www.w3.org/2000/svg">
+      <path
+        fillRule="evenodd"
+        clipRule="evenodd"
+        d="M4 4H4.5L7 4L13 4L13.2071 4L13.3536 4.14645L15.8536 6.64645L16 6.79289V7V11.3773C15.6992 11.1362 15.3628 10.9376 15 10.7908V7.20711L13.5 5.70711V7V7.5L13 7.5L7 7.5L6.5 7.5V7V5L5 5V15H6.5V10V9.5H7H13H13.5V10V10.5C13.1547 10.5 12.8196 10.5438 12.5 10.626V10.5H7.5V15H9.53095C9.57451 15.3493 9.66311 15.6847 9.79076 16H7H4.5H4V15.5V4.5V4ZM7.5 5V6.5L12.5 6.5V5L7.5 5ZM16.3904 14.1877L14.3904 11.6877L13.6096 12.3123L14.9597 14H11V15H14.9597L13.6096 16.6877L14.3904 17.3123L16.3904 14.8123L16.6403 14.5L16.3904 14.1877Z"
+        fill="currentColor"
+      />
+    </svg>
+  ),
   folder: (
     <svg viewBox="0 0 20 20" fill="none" xmlns="http://www.w3.org/2000/svg">
       <path