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/preload.js

@@ -34,6 +34,7 @@ const api = {
     add: invoke('step:add'),
     save: invoke('step:save'),
     delete: invoke('step:delete'),
+    restore: invoke('step:restore'),
     reorder: invoke('steps:reorder'),
     imagePath: invoke('step:imagePath'),
     setWorkingImage: invoke('step:setWorkingImage'),