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

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

src/lib/commandBarConfigs/modelingCommandConfig.ts

@@ -88,7 +88,7 @@ export const modelingMachineCommandConfig: StateMachineCommandSetConfig<
   ],
   Export: {
     description: 'Export the current model.',
-    icon: 'exportFile',
+    icon: 'floppyDiskArrow',
     needsReview: true,
     args: {
       type: {