Totals calculation features / fixes

In some situation you only need to show total count of rows in grid and don't need to calculate totals for any column (field). In such case before all fields was automatically added from model. Now we can pass addTotals(null) or addTotals(false) and get what we need - only count records, but no totals for fields will be calculated.

From 4.2 branch: https://github.com/atk4/atk4/commit/9313c94fc4b4ec18633a7760348d5fae3b6931e8