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
Merge pull request #542 from DarkSide666/patch-3

Totals calculation features / fixes