Use QQuickWidget in welcome mode when compiling with Qt 5.5 or later.

Several bugs were fixed in QQuickWidget, making it viable to use it by
default with Qt 5.5.

Change-Id: If31d2cfabf80d01b1588269a1fde96742621e995
Task-number: QTCREATORBUG-13440
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>

src/plugins/welcome/welcome.pro

 
 QT += quick
 
-!isEmpty(USE_QUICK_WIDGET) {
-    QT +=  quickwidgets
-    DEFINES += USE_QUICK_WIDGET
-}
-
 QML_IMPORT_PATH=../../../share/qtcreator/welcomescreen
 
 include(../../qtcreatorplugin.pri)
 
+!isEmpty(USE_QUICK_WIDGET)|minQtVersion(5, 5, 0) {
+    QT +=  quickwidgets
+    DEFINES += USE_QUICK_WIDGET
+}
+
 HEADERS += welcomeplugin.h
 
 SOURCES += welcomeplugin.cpp

src/plugins/welcome/welcomeplugin.cpp

@@ -215,7 +215,7 @@ void WelcomeMode::facilitateQml(QQmlEngine *engine)
 
     ctx->setContextProperty(QLatin1String("creatorTheme"), &m_themeProperties);
 
-#if defined(USE_QUICK_WIDGET)
+#if defined(USE_QUICK_WIDGET) && (QT_VERSION < QT_VERSION_CHECK(5, 5, 0))
     bool useNativeText = !HostOsInfo::isMacHost();
 #else
     bool useNativeText = true;