Live preview: the tool alone, the boolean only on OK, computed off the window thread (#1201)

The preview builds only the tool; the boolean runs once on OK, off the window thread, with a Computing label while it takes a while. Follow-up to #1020.

assets/locales/ja.json

@@ -286,6 +286,7 @@
   "cmd.view_list": "名前付きビューを一覧表示",
   "cmd.view_rename": "名前付きビューの名前を変更",
   "cmd.view_save": "新規名前付きビュー",
+  "dialog.computing": "計算中… {seconds} 秒",
   "dialog.custom_axis": "↖ カスタム軸",
   "dialog.edit_title": "{title}を編集",
   "dialog.select": "↖ 選択",