QmlDesigner: Hide target buttons for centered anchoring

I never ever needed that feature and the functionality is
missing/incomplete.
Setting all anchor combinations for the left/right/top/bottom
anchors should be enough.

Change-Id: I2c1a165ff7ef0ecc666640aeba7e86166c90aa25
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

share/qtcreator/qmldesigner/propertyEditorQmlSources/QtQuick/AnchorRow.qml

@@ -56,6 +56,8 @@ RowLayout {
     signal centerButtonClicked
     signal oppositeEdgeButtonClicked
 
+    property alias buttonRow: buttonRow
+
 
     IconLabel {
         id: icon

share/qtcreator/qmldesigner/propertyEditorQmlSources/QtQuick/LayoutSection.qml

@@ -152,6 +152,7 @@ Section {
                 anchorBackend.horizontalTarget = currentText
             }
             verticalAnchor: false
+            buttonRow.visible: false
         }
 
         AnchorRow {
@@ -164,6 +165,7 @@ Section {
                  anchorBackend.verticalTarget = currentText
             }
             verticalAnchor: true
+            buttonRow.visible: false
         }
 
     }