"Phantom" header row remains after resetting QTableView.

When the model backing a QTableView resets and in the process loses all row/columns, an unsightly grey stripe sometimes remains where the vanished headers were. Initially the view does not show this, and sometimes odd things (like changing tabs) will heal it.

The bug appears to be that the call stack QHeaderView::reset()>initializeSections()>clear() (the case when columnCount() is 0) does not invalidate the cached size hint. Thus, a horizontal header view with d->sectionCount=0 (which does not paint any headers) continues to take up however much height it did before being reset, until something unrelated invalidates the cached size hint, at which point it recomputes to 0 and disappears properly.

See attached repro app for an isolated testcase: the view begins empty, when you hit fill a column appears, when you hit empty it disappers but continues the horizontal header continues to take up space, if you flip between the tabs the spurious header will go away (but comes back if you hit fill and empty again).

Possible fix could be as simple as adding invalidateCachedSizeHint() to QHeaderViewPrivate::clear().