Sort between different object types with same path

[Speedy37]

The current implementation of the sort is strict: if a query result in a list of two kind of objects, any sort on that list must be on the exact same attributes and orders.

It's possible to relax this restriction by saying:

A is declared as A { a, b, c }
B is declared as B { a, d, c }

A: { '.': ['+a', '+b'] },
B: { '.': ['+a'] },
is valid, and mean order by "a asc" then by "b asc" if b exists otherwise consider b value as being an empty string or zero depending of the type.

A: { '.': ['+a', '+b', '+c'] },
B: { '.': ['+a', '+c'] },
is valid and mean order by a, then b if any, then c.

The rules for an attribute to differ would be:

• it can't be the first attribute (to prevent common mistakes)
• it can't exists on the other type (to prevent common mistakes)