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>

src/widgets/styles/qstyle.cpp

@@ -1750,6 +1750,9 @@ void QStyle::drawItemPixmap(QPainter *painter, const QRect &rect, int alignment,
     \value SH_ComboBox_Popup  Allows popups as a combobox drop-down
         menu.
 
+    \omitvalue SH_ComboBox_UseNativePopup  Whether we should use a native popup.
+        Only supported for non-editable combo boxes on Mac OS X so far.
+
     \value SH_Workspace_FillSpaceOnMaximize  The workspace should
         maximize the client area.
 

src/widgets/styles/qstyle.h

@@ -702,6 +702,7 @@ public:
         SH_ToolTip_FallAsleepDelay,
         SH_Widget_Animate,
         SH_Splitter_OpaqueResize,
+        SH_ComboBox_UseNativePopup,
         // Add new style hint values here
 
         SH_CustomBase = 0xf0000000