Added a entry on DatabaseModel::getObjectDependecies() to get the
sequence that a column depends on. (issue #480)

Fixed a bug ColumnWidget class that was wrongly removing the sequence
assigned to a column. Additional fix for issue #480
Minor fix on ObjectDepsRefsWidget that was triggering an error when
trying to open a table child object from there.
Merge branch 'column-sequence-fix' into develop