This fixes a defect where station detail changes are not saved.
The Qt sort and filter proxy models utilize an item cache that must be
flushed by callig submit() before accessing the underlying model if
the proxy model has been used for updates.
Also separated the item model candidate key filter from the
implementation internals of the foreign key item delegate so that
candidate key filtered models can be used directly as view models.
Make the insert new station details band combo box use a candidate key
filtered item model to avoid constraint violations. Constraint is zero
or one station records per band.
git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@5161 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
