Add Marketplace plug-in, starting a rewrite from the Toolbox

This plug-in will be a complete re-write of the previous Toolbox plug-in. It's intended to solve some of the inherent architectural problems with the Toolbox.
We're calling it Marketplace now as well.

Contributes to issue CURA-8556.

plugins/Marketplace/Marketplace.py

@@ -0,0 +1,10 @@
+# Copyright (c) 2021 Ultimaker B.V.
+# Cura is released under the terms of the LGPLv3 or higher.
+
+from UM.PluginObject import PluginObject
+
+class Marketplace(PluginObject):
+    """
+    The main managing object for the Marketplace plug-in.
+    """
+    pass  # TODO

plugins/Marketplace/__init__.py

@@ -0,0 +1,17 @@
+# Copyright (c) 2021 Ultimaker B.V.
+# Cura is released under the terms of the LGPLv3 or higher.
+
+from .Marketplace import Marketplace
+
+def getMetaData():
+    """
+    Extension-type plug-ins don't have any specific metadata being used by Cura.
+    """
+    return {}
+
+
+def register(app):
+    """
+    Register the plug-in object with Uranium.
+    """
+    return { "extension": Marketplace() }

plugins/Marketplace/plugin.json

@@ -0,0 +1,8 @@
+{
+    "name": "Marketplace",
+    "author": "Ultimaker B.V.",
+    "version": "1.0.0",
+    "api": 7,
+    "description": "Manages extensions to the application and allows browsing extensions from the Ultimaker website.",
+    "i18n-catalog": "cura"
+}