Styles: improve SH_Header_ArrowAlignment handling

The docs were wrong, and the returned values from some styles
did not match reality, so fix that. Again, this style hint was
not used at all within QWidgets...

Task-number: QTBUG-629
Change-Id: Ie6ff80fd09bc3292ba3d787ccca4d6f4c0056e89
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>

src/widgets/styles/qcommonstyle.cpp

@@ -4907,10 +4907,13 @@ int QCommonStyle::styleHint(StyleHint sh, const QStyleOption *opt, const QWidget
 
 
     case SH_TabBar_Alignment:
-    case SH_Header_ArrowAlignment:
         ret = Qt::AlignLeft;
         break;
 
+    case SH_Header_ArrowAlignment:
+        ret = Qt::AlignRight | Qt::AlignVCenter;
+        break;
+
     case SH_TitleBar_AutoRaise:
         ret = false;
         break;

src/widgets/styles/qstyle.cpp

@@ -1663,7 +1663,8 @@ void QStyle::drawItemPixmap(QPainter *painter, const QRect &rect, int alignment,
 
     \value SH_Header_ArrowAlignment The placement of the sorting
         indicator may appear in list or table headers. Possible values
-        are Qt::Left or Qt::Right.
+        are Qt::Alignment values (that is, an OR combination of
+        Qt::AlignmentFlag flags).
 
     \value SH_Slider_SnapToValue  Sliders snap to values while moving,
         as they do on Windows.

src/widgets/styles/qwindowsvistastyle.cpp

@@ -2112,6 +2112,9 @@ int QWindowsVistaStyle::styleHint(StyleHint hint, const QStyleOption *option, co
         else
             ret = -1;
         break;
+    case SH_Header_ArrowAlignment:
+        ret = Qt::AlignTop | Qt::AlignHCenter;
+        break;
     default:
         ret = QWindowsXPStyle::styleHint(hint, option, widget, returnData);
         break;