(core) Fix widget loading spinner title

Summary:
The loading spinner would always display
'Building Table widget' when creating or changing
a widget. This fixes the title to reflect the selected
widget type.

Test Plan:
Updated existing browser tests to verify the loading spinner
title includes the correct widget type.

Reviewers: dsagal, paulfitz

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2863

app/client/ui2018/modals.ts

@@ -212,7 +212,10 @@ export async function spinnerModal<T>(
 
     return [
       cssModalSpinner.cls(''),
-      cssModalTitle(title),
+      cssModalTitle(
+        title,
+        testId('modal-spinner-title'),
+      ),
       cssSpinner(loadingSpinner()),
       testId('modal-spinner'),
     ];