Fix the window flags for Cocoa windows

This ensures that the possible window flag combinations are respected
where possible in Cocoa.

Task-number: QTBUG-34645
Task-number: QTBUG-31616
Change-Id: I6be8ca666b7cbc397575e97cd95ea298f52a7113
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
QFileSelector: mark ctor explicit

This is standard for (QObject*) ctors.

Change-Id: I4756ba50b1f3148d72e95e581d52a37ebd47a7ae
Reviewed-by: John Layt <jlayt@kde.org>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Crash fix in QTreeView::sizeHintForColumn().

Vertical scrollbar may get out of sync. When this happens, the calculation of
firstVisibleItem will retrun "-1". This must be handled in ::sizeHintForColumn().
Added an auto-test for the crashes.

Task-number: QTBUG-34717

Change-Id: I867fd144ef3ce45e382337c5eafe345f573cd944
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>