* support for count and aggregations on columns, close#786
* support explain query for aggregations
* '<arr-rel>_agg' in '<table>' type, fix order by for aggregations
* add 'allow_aggregations' key in select permissions
* Add checkbox to toggle count and aggregations on columns on select permission
* align aggregation checkbox with columns div
* improve readability of the generated sql
* alias is needed at the top level aggregation
* throw internal errors for unexpected fields
* rename SelFld to more readable TableAggFld
* rename agg to aggregate
