Add function to open Marketplace window

This will now load in a QML file, cache it, and create an empty window with the title 'Marketplace'.

Contributes to issue CURA-8556.

plugins/Marketplace/Marketplace.py

@@ -1,10 +1,41 @@
 # Copyright (c) 2021 Ultimaker B.V.
 # Cura is released under the terms of the LGPLv3 or higher.
 
-from UM.PluginObject import PluginObject
+import os.path
+from PyQt5.QtCore import pyqtSlot
+from typing import Optional, TYPE_CHECKING
 
-class Marketplace(PluginObject):
+from cura.CuraApplication import CuraApplication  # Creating QML objects and managing packages.
+from UM.Extension import Extension  # We are implementing the main object of an extension here.
+from UM.Logger import Logger
+from UM.PluginRegistry import PluginRegistry  # To find out where we are stored (the proper way).
+
+if TYPE_CHECKING:
+    from PyQt5.QtCore import QObject
+
+class Marketplace(Extension):
     """
     The main managing object for the Marketplace plug-in.
     """
-    pass  # TODO
+
+    def __init__(self):
+        super().__init__()
+        self._window: Optional["QObject"] = None  # If the window has been loaded yet, it'll be cached in here.
+
+    @pyqtSlot()
+    def show(self) -> None:
+        """
+        Opens the window of the Marketplace.
+
+        If the window hadn't been loaded yet into Qt, it will be created lazily.
+        """
+        if self._window is None:
+            plugin_path = PluginRegistry.getInstance().getPluginPath(self.getPluginId())
+            if plugin_path is None:
+                plugin_path = os.path.dirname(__file__)
+            path = os.path.join(plugin_path, "resources", "qml", "Marketplace.qml")
+            self._window = CuraApplication.getInstance().createQmlComponent(path, {})
+        if self._window is None:  # Still None? Failed to load the QML then.
+            Logger.error(f"Failed to load QML for Marketplace window.")
+            return
+        self._window.show()

plugins/Marketplace/resources/qml/Marketplace.qml

@@ -0,0 +1,21 @@
+// Copyright (c) 2021 Ultimaker B.V.
+// Cura is released under the terms of the LGPLv3 or higher.
+
+import QtQuick 2.15
+import QtQuick.Window 2.2
+
+import UM 1.2 as UM
+
+Window
+{
+    id: marketplaceDialog
+    property variant catalog: UM.I18nCatalog { name: "cura" }
+
+    minimumWidth: UM.Theme.getSize("modal_window_minimum").width
+    minimumHeight: UM.Theme.getSize("modal_window_minimum").height
+    width: minimumWidth
+    height: minimumHeight
+
+    title: "Marketplace" //Seen by Ultimaker as a brand name, so this doesn't get translated.
+    modality: Qt.NonModal
+}