Add a StackView around Marketplace to allow extra pages on top

This allows a sort of full-screen pop-up to replace the entire Marketplace window contents, on top of the normal contents. The normal contents are kept as they are, but out of view.

Contributes to issue CURA-8565.

plugins/Marketplace/resources/qml/PackageDetails.qml

@@ -0,0 +1,10 @@
+// Copyright (c) 2021 Ultimaker B.V.
+// Cura is released under the terms of the LGPLv3 or higher.
+
+import QtQuick 2.15
+import QtQuick.Controls 2.15
+
+Label
+{
+    text: "Test!"
+}