Move grid total function definition to grid

Currently we have automatic grid totals calculation functionality. To activate it you need:

• totals_render_as="grid_total_row" parameter in "m_RenderElement" tag, that renders "grid" block
• add "totals" key (possible values are SUM, COUNT, AVG or any MySQL aggregate function) in field declaration in "Fields" property in unit config

Such approach works, but won't allow to have different totals function based on place, where grid is used or special-based.

I propose to move "totals" option to field declaration in grid definition part, where "filter_block" is defined.