QmlDesigner: Fix crash building against Qt 5.15

QGradient construction has changed in 5.15 so that it will no longer
allow creating a gradient for the final enum that indicates the number
of presets rather than actual preset.

Change-Id: I5304b924890f8ae4530834a2b34062e789147c5d
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/plugins/qmldesigner/components/propertyeditor/gradientpresetdefaultlistmodel.cpp

@@ -56,6 +56,8 @@ void GradientPresetDefaultListModel::addAllPresets()
         return;
 
     for (int i = 0; i < metaEnum.keyCount(); i++) {
-        addItem(GradientPresetItem(GradientPresetItem::Preset(metaEnum.value(i))));
+        auto preset = GradientPresetItem::Preset(metaEnum.value(i));
+        if (preset < GradientPresetItem::Preset::NumPresets)
+            addItem(GradientPresetItem(preset));
     }
 }