QTreeView keyboardSearch may match with first column instead of current

This description references 4.7.4 specifically, but I could reproduce the bug in 4.8.2.

When trying to seek to a row in a QTreeView, input is sometimes matched with data from the first column even though the current index may have a different column.

In one case, the selected index was on column 2 and I expected the seek to occur there. But the call to indexBelow (at gui/itemviews/qtreeview.cpp:987) returned a QModelIndex with column 0 while the `start` argument was at column 2. Then the input was matched on column 0 and the current index was changed to that column. I expected the current index to not change columns.