Auto-scroll while selecting entire rows/columns did not work

Auto-scroll while selecting entire rows/columns did not work

If you press and hold a section in a header view you can extend the
selection to more rows by moving the mouse. This worked fine until you
moved the mouse outside the geometry of the header view. The expected
behavior was then to scroll the view (this is what happens with extended
selections on regular table cells).

[ChangeLog][QtWidgets][QHeaderView] Auto-scroll the view when making
extended row/column selections.

Change-Id: Ic65aa34d370e74054b2123ab57edb1add0e8adb9
Task-number: QTBUG-21201
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>
(cherry picked from qtbase/f9408317e70bc2e635a2f9baeff35d1c06227734)