add Product function to Subtotals UI

Change-Id: I240859bafa953c75ee639ccb77305161274217d1
Reviewed-on: https://gerrit.libreoffice.org/15210Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sc/uiconfig/scalc/ui/subtotalgrppage.ui

@@ -23,6 +23,9 @@
       <row>
         <col id="0" translatable="yes">Min</col>
       </row>
+      <row>
+        <col id="0" translatable="yes">Product</col>
+      </row>
       <row>
         <col id="0" translatable="yes">Count (numbers only)</col>
       </row>