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/renderer/app.js

@@ -279,7 +279,7 @@ class StepForgeApp {
         }
         send({ action: s.paused ? 'resume' : 'pause' });
       },
-    }, notStarted ? 'Start recording' : s.paused ? 'Resume' : 'Pause');
+    }, s.paused ? 'Start recording' : 'Stop recording');
 
     this.captureStatus.append(
       el('span', { title: `Capture session — ${trigger}` }, `Rec - ${trigger}`),