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/de.json

@@ -10,6 +10,7 @@
   "browser.precision": "Genauigkeit: {precision}",
   "browser.sketch_name": "Skizze {number}",
   "browser.units": "Einheiten: {units}",
+  "dialog.computing": "Wird berechnet… {seconds} s",
   "dialog.custom_axis": "↖ Eigene Achse",
   "dialog.edit_title": "{title} bearbeiten",
   "dialog.select": "↖ Auswählen",