QComboBox: Use native popups on Mac

This remains an opt-in solution bound to the usage
of SH_ComboBox_UseNativePopup in a proxy style. The
midterm goal is to make this option on by default,
possibly in 5.4. This solution is and will remain a hint
in the sense that some exotic use cases of QComboBox
(e.g., when setting its view) are inherently incompatible
with the native popup idea.

Task-number: QTBUG-32731
Change-Id: I2a3d780795c22f9989e44325fcaf314538b1de49
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
QStyle: Add new SH_ComboBox_UseNativePopup style hint

If the style hints it, QComboBox::showPopup() will try to
show a native popup instead. This is currently undocumented
and an opt-in feature (typically by using a proxy style).

Works only on Mac so far.

Task-number: QTBUG-32731
Change-Id: I4447e884cbd6b490f3039c7a95168698c0bed16e
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
in command line args, escape tabs as well

Task-number: QTBUG-27154
Change-Id: I4a204b2bf2231027db55a444f304190c3b30878c
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
network: finish all pending replies upon error

... and not only one. This was a problem e.g. when there were several
requests to the same host and the host was not reachable; only one
reply would get an error signal in case we suppressed other errors in
"happy eyeballs" host lookup style.

Task-number: QTBUG-36890

Change-Id: I1b5757498bd644b0d773cf6c43e4950620949c5c
Reviewed-by: Richard J. Moore <rich@kde.org>