Switch pages to Materials when tab is clicked

And back to plug-ins when that tab is clicked.
Sadly, linking the content dynamically doesn't seem to work, with a custom property.

Contributes to issue CURA-8557.

plugins/Marketplace/resources/qml/Materials.qml

@@ -0,0 +1,11 @@
+// Copyright (c) 2021 Ultimaker B.V.
+// Cura is released under the terms of the LGPLv3 or higher.
+
+import Marketplace 1.0 as Marketplace
+
+Packages
+{
+    model: Marketplace.PackageList
+    {
+    }
+}