Show either openFileButton or openFileMenu, depending on provider count

If there's just 1 provider, show the button. If there are multiple, show the menu. If there are 0, also show the button but disable the button.
The behaviour is not yet implemented though. It doesn't actually look at which providers are available and what they do.

Contributes to issue CURA-8008.

plugins/PrepareStage/PrepareMenu.qml

@@ -13,6 +13,8 @@ Item
 {
     id: prepareMenu
 
+    property var fileProviderModel: CuraApplication.getFileProviderModel()
+
     UM.I18nCatalog
     {
         id: catalog
@@ -78,6 +80,8 @@ Item
         Cura.ExpandablePopup
         {
             id: openFileMenu
+            visible: prepareMenu.fileProviderModel.count > 1
+
             contentAlignment: Cura.ExpandablePopup.ContentAlignment.AlignLeft
             headerCornerSide: Cura.RoundedRectangle.Direction.All
             headerPadding: Math.round((parent.height - UM.Theme.getSize("button_icon").height) / 2)
@@ -107,11 +111,12 @@ Item
         Button
         {
             id: openFileButton
+            visible: prepareMenu.fileProviderModel.count <= 1
 
             height: parent.height
-            width: height //Square button.
+            width: visible ? height : 0 //Square button (and don't take up space if invisible).
             onClicked: Cura.Actions.open.trigger()
-            enabled: visible
+            enabled: visible && prepareMenu.fileProviderModel.count > 0
             hoverEnabled: true
 
             contentItem: Item