[fix #11475] expressions: add left and right associavity for dump of binary operators

Previous fix  2c7378bb16f7e6433363340b9c16e40c3ad654ae solved original bug by assuming that all operators were left associative, which is not true for power operator.

This introduces left and right associativity in the dump.
This fixes the bug and I suppose will remove a bit more brackets.

see http://hub.qgis.org/issues/11475
isn't a bit more future proof?
if you add a new operator you'll have to complete the method to avoid an error.