QTableView stored the current row/column selection start in an own
variable instead using currentSelectionStartIndex. This leads to an
inconsistent behavior when the selection is done with a click on the
header and then in a cell (and the other way round)
Fixes: QTBUG-92561
Change-Id: I4c8bda3a938de451b6eff2819141e86a6870fbef
Pick-to: 6.1 6.2
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>