improve Query Builder to make more complex queries

I like your query grouping idea. Personally, I wouldn't mind seeing more streamlining across the database engine like this, but hand my hands full with everything else going on I didn't want to touch anything like that until the framework got to an official release. And I had enough problems with the db refactoring I did do that I didn't want to touch that whole layer anymore than I had to back then.

And a few more questions.
What to do with the old version of the query grouping (group_start(), etc.)?
Leave for backward compatibility or remove completely?

For query grouping, create a new PR or update an existing one to avoid merge conflicts?