• this fixes sibling() - the unittest confirmed Dan's assessment that

sibling() got broken for StatisticsProxyModel with Qt5.
And this finally repairs the browser tab in akonadiconsole, clicking
on a folder lists its contents.

• this reduces the number of dataChanged signals emitted by the proxy:

instead of dataChanged((row,0) (row,0)) + dataChanged((row,1) (row,3))
the proxy now emits dataChanged((row,0) (row,3)) directly

• I also removed the code emitting dataChanged for parent indexes,

the reasoning was "so that cumulative totals can be updated", but there
are no cumulative totals anywhere in this proxy (other than the tooltip),
although I now see that the delegate is where cumulative totals happen...

• the awful model-index-struct-casting hack is no longer necessary,

the code is much simpler, there are now unittests (both here and for
the base class KExtraColumnsProxyModel in kitemmodels)...

unittest + akonadiconsole + kmail
Deleting a mail in akonadiconsole updates the total in the closed parent
folder in kmail's foldertree. So that still works.
I wonder if I'm missing a case where the delegate needs the proxy to
emit dataChanged for parents so that they are refreshed correctly, though.