Qt5: fix build with Qt 5.11

And while at it annotate Qt5Font fall-throughs.

Change-Id: I31a73407f3228f303236983df17c66bc88eef6c5
Reviewed-on: https://gerrit.libreoffice.org/61557
Tested-by: Jenkins
Tested-by: Rene Engelhard <rene@debian.org>
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>

vcl/qt5/Qt5AccessibleWidget.cxx

@@ -116,8 +116,8 @@ void lcl_appendRelation(QVector<QPair<QAccessibleInterface*, QAccessible::Relati
     for (sal_uInt32 i = 0; i < nTargetCount; i++)
     {
         Reference<XAccessible> xAccessible(aRelation.TargetSet[i], uno::UNO_QUERY);
-        relations->append(QPair(
-            QAccessible::queryAccessibleInterface(new Qt5XAccessible(xAccessible)), aQRelation));
+        relations->append(
+            { QAccessible::queryAccessibleInterface(new Qt5XAccessible(xAccessible)), aQRelation });
     }
 }
 }

vcl/qt5/Qt5Font.cxx

@@ -35,8 +35,11 @@ static QFont::Weight GetQFontWeight(FontWeight eWeight)
             return QFont::Light;
         case FontWeight_FORCE_EQUAL_SIZE:
             assert(false && "FontWeight_FORCE_EQUAL_SIZE not implementable for QFont");
+            SAL_FALLTHROUGH;
         case WEIGHT_SEMILIGHT:
+            SAL_FALLTHROUGH;
         case WEIGHT_DONTKNOW:
+            SAL_FALLTHROUGH;
         case WEIGHT_NORMAL:
             return QFont::Normal;
         case WEIGHT_MEDIUM: