QmlDesigner.PropertyEditor: Use DesignerScrollViewStyle

I set corner to an empty item, because otherwise it looks weird if both
scrollbars are visible.

Change-Id: I149e2ff58f9e9f71b2bb9f435fd64388cbf5529c
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>

share/qtcreator/qmldesigner/common/DesignerScrollViewStyle.qml

@@ -58,4 +58,5 @@ ScrollViewStyle {
 
     decrementControl: Item {}
     incrementControl: Item {}
+    corner: Item {}
 }

share/qtcreator/qmldesigner/propertyEditorQmlSources/HelperWidgets/ScrollView.qml

 import QtQuick 2.1
 import QtQuick.Controls 1.0 as Controls
+import "../../common/"
 
 Controls.ScrollView {
+    style: DesignerScrollViewStyle {
 
+    }
+    frameVisible: false
 }