Improve time to the top of the stash being visible

Load just the immediate children of the stash root first, so that we
load sub-trees under each child folder-by-folder.  This allows the UI to
draw the folders that show up near the top of the stash while the
folders further down are still being loaded.

src/model/bookmarks.ts

@@ -360,6 +360,7 @@ export class Model {
    * the stash. */
   async loadedStash(): Promise<LoadedFolder | undefined> {
     if (!this.stash_root.value) return;
+    await this.loaded(this.stash_root.value);
     return await this.loadedSubtree(this.stash_root.value);
   }