Add support for showing the object count in the live table

The use case for this is: you have a live table that shows the pages that have objects of a given type.

#set ($options = {
  'className': 'Path.To.MyClass',
  ...
  'queryFilters': ['unique']
})

A page can have multiple objects of this type ('Path.To.MyClass') so you use the unique query filter to make sure you don't have duplicated rows. But then you want to know how many objects there are on each listed page.

#set ($columnsProperties = {
  ...
  'doc.objectCount': {'type': 'numeric', 'filterable': false, 'sortable': false},
  ...
})