Revert "Use persistent model index in devices widget"

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

client/deviceswidget.h

@@ -58,7 +58,7 @@ private:
     void setDeviceInfoButtonsVisible(bool show);
 
     PortGroupList *portGroups_;
-    QPersistentModelIndex currentPortIndex_;
+    QModelIndex currentPortIndex_;
 };
 
 #endif