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/en.json

@@ -286,6 +286,7 @@
   "cmd.view_list": "List Named Views",
   "cmd.view_rename": "Rename Named View",
   "cmd.view_save": "New Named View",
+  "dialog.computing": "Computing… {seconds} s",
   "dialog.custom_axis": "↖ Custom Axis",
   "dialog.edit_title": "EDIT {title}",
   "dialog.select": "↖ Select",