Improved the way many-to-many rels. can be configured. Now the generated
table can have a single column as pk or a multi-value pk.

The ModelWidget::convertRelationshipNN method was improved in order to
correctly convert many-to-many rels with single or multi-valued pks.

A new name patter was introduced PK_COL_PATTERN which is used to
generate the single pk column name for many-to-many relationships.
Merge branch 'rel-nn-improvements' into model-db-sync