Overridet totalFilteredItemsLength when useExternalFilter == true

The grid show the wrong amount of filtered items when you filter server side. The computed observable totalFilteredItemsLength needs to check useExternalFilter and then use a supplied variabel on the filterOptions to populate that info

Start by locating the computed observable totalFilteredItemsLength and the useExternalFilter handling in the grid code. Read how filterOptions is populated for server-side filtering, then verify that the supplied total is displayed as the filtered-item count when external filtering is enabled.

Written by the indexing model from the issue text.