Align sizes of prepare and preview menus

For some reason, the prepare menu is still one pixel too small. I think it's got to do with that one being a layout rather than a normal row.

Contributes to issue CURA-8202.

plugins/PrepareStage/PrepareMenu.qml

@@ -23,16 +23,14 @@ Item
     {
         left: parent.left
         right: parent.right
-        leftMargin: UM.Theme.getSize("wide_margin").width
-        rightMargin: UM.Theme.getSize("wide_margin").width
+        leftMargin: UM.Theme.getSize("wide_margin").width * 2
+        rightMargin: UM.Theme.getSize("wide_margin").width * 2
     }
 
     // Item to ensure that all of the buttons are nicely centered.
     Item
     {
-        anchors.horizontalCenter: parent.horizontalCenter
-        width: parent.width - 2 * UM.Theme.getSize("wide_margin").width
-        height: parent.height
+        anchors.fill: parent
 
         RowLayout
         {

plugins/PreviewStage/PreviewMenu.qml

@@ -24,17 +24,15 @@ Item
     {
         left: parent.left
         right: parent.right
-        leftMargin: UM.Theme.getSize("wide_margin").width
-        rightMargin: UM.Theme.getSize("wide_margin").width
+        leftMargin: UM.Theme.getSize("wide_margin").width * 2
+        rightMargin: UM.Theme.getSize("wide_margin").width * 2
     }
 
     Row
     {
         id: stageMenuRow
 
-        anchors.horizontalCenter: parent.horizontalCenter
-        width: parent.width - 2 * UM.Theme.getSize("wide_margin").width
-        height: parent.height
+        anchors.fill: parent
         // This is a trick to make sure that the borders of the two adjacent buttons' borders overlap. Otherwise
         // there will be double border (one from each button)
         spacing: -UM.Theme.getSize("default_lining").width
@@ -52,7 +50,7 @@ Item
         {
             id: viewPanel
             height: parent.height
-            width: source != "" ? (previewMenu.width - viewsSelector.width - printSetupSelectorItem.width) : 0
+            width: source != "" ? (parent.width - viewsSelector.width - printSetupSelectorItem.width) : 0
             source: UM.Controller.activeView != null && UM.Controller.activeView.stageMenuComponent != null ? UM.Controller.activeView.stageMenuComponent : ""
         }