This reverts commit 46dd028a33.
The above commit caused a regression - issue #281
As seen in valgrind, crash was caused by portModel reset writing into
a freed block - presumably pertaining to persistent indexes. The block
was freed by the persistent index itself earlier when it was set to
invalid - so the model should not have been accessing that.
The application code seems correct - maybe the Qt code has a bug?
Unlikely but not impossible. For now go back to using QModelIndex
instead of QPersistentModelIndex
Fixes#281
