When a new model was set with setSourceModel() and the mapping was
built up, the destruction of the old model caused a reset in the
QSortFilterProxyModel which lead to an empty view or an assertion.
Now we properly disconnect the old model again and also clean up the old
mapping/persistent indexes when a new source model is set.
Task-number: QTBUG-44962
Task-number: QTBUG-67948
Task-number: QTBUG-68427
Change-Id: I2e0612899c210bde3ac0cfa59aefd78269deee5b
Reviewed-by: David Faure <david.faure@kdab.com>