Allow open file button to resize depending on content items

This re-links the widths, heights and paddings such that we can later change the width of the button depending on its contents.
However there is a complication: Buttons automatically change the size of the contents based on the size of the button minus its padding. So making the size of the button in turn depend on its contents causes a binding loop.
To get around this binding loop, we need to manually calculate the size of the button, such that the size of the contents ends up exactly right.

Contributes to issue CURA-8008.

plugins/PrepareStage/PrepareMenu.qml

@@ -77,14 +77,25 @@ Item
         Button
         {
             id: openFileButton
+
+            //Make the button square if the contents are.
+            leftPadding: topPadding
+            rightPadding: topPadding
+            bottomPadding: topPadding
+
             height: UM.Theme.getSize("stage_menu").height
-            width: UM.Theme.getSize("stage_menu").height
+            width: openFileIconContainer.width + leftPadding + rightPadding
             onClicked: Cura.Actions.open.trigger()
             hoverEnabled: true
 
-            contentItem: Item
+            contentItem: Row
             {
-                anchors.fill: parent
+                Item
+                {
+                    id: openFileIconContainer
+                    height: parent.height
+                    width: height //Square button.
+
                     UM.RecolorImage
                     {
                         id: buttonIcon
@@ -97,6 +108,7 @@ Item
                         sourceSize.height: height
                     }
                 }
+            }
 
             background: Rectangle
             {