Allow fully custom query

[ext]

While rudimentary grouping and joining is possible with BasicObject::selection there are many cases where it just isn't possible to generate the needed SQL. For instance joining the table with itself needs table aliases which BO doesn't add. Grouping in general isn't BOs strong side but using a HAVING condition is just not possible.

This change allows for using a fully customized query (grouping, joining, etc) while still creating instances of your BO model similar to how BasicObject::selection does.

static public function myCustomSelection(){
  return static::selection_execute([
    'SELECT * FROM `my_table` ...'
  ]);
}

It assumes the user know what he is doing (i.e. not selecting all the required fields it considered a programming error by the caller).