- rework of Query._adapt_clause to support [ticket:2155],

- rework of Query._adapt_clause to support [ticket:2155],
be clearer in its intent.
- Fine tuning of Query clause adaptation when
from_self(), union(), or other "select from
myself" operation, such that plain SQL expression
elements added to filter(), order_by() etc.
which are present in the nested "from myself"
query *will* be adapted in the same way an ORM
expression element will, since these
elements are otherwise not easily accessible.
[ticket:2155]