Add global undo for step deletion; simplify capture session UI

- The canvas Undo/Redo buttons now also undo/redo step deletion
  (single or multi-select), restoring the step's data, images, and
  position in the order. Backed by a new step:restore IPC call and
  GuideStore.restoreStep().
- Remove "Finish capture session" from the capture menu. The
  top-right recording bar in the guide editor is now the only place
  to start/stop recording, with its toggle relabeled
  "Start recording" / "Stop recording".

app/main.js

@@ -419,6 +419,15 @@ function setupIpc() {
     reindex(guideId);
     return true;
   });
+  h('step:restore', ({ guideId, step, originalBase64, workingBase64, position }) => {
+    const images = {
+      original: originalBase64 ? Buffer.from(originalBase64, 'base64') : null,
+      working: workingBase64 ? Buffer.from(workingBase64, 'base64') : null,
+    };
+    const restored = store.restoreStep(guideId, step, images, position);
+    reindex(guideId);
+    return restored;
+  });
   h('steps:reorder', ({ guideId, order }) => store.reorderSteps(guideId, order));
   h('step:imagePath', ({ guideId, stepId, which }) => {
     const p = store.stepImagePath(guideId, stepId, which || 'working');