High-level layout of Marketplace window

Currently just a title and a page. The title is separate because in between there will be the tabs selecting which page is shown. The title will also change depending on that, but that'll have to be implemented separately. The page is loaded with a loader to make it efficient, and also to make it extensible when the tabs get implemented in a follow-up ticket.

Contributes to issue CURA-8556.

plugins/Marketplace/resources/qml/Marketplace.qml

@@ -2,6 +2,8 @@
 // Cura is released under the terms of the LGPLv3 or higher.
 
 import QtQuick 2.15
+import QtQuick.Controls 2.15
+import QtQuick.Layouts 1.15
 import QtQuick.Window 2.2
 
 import UM 1.2 as UM
@@ -18,4 +20,35 @@ Window
 
     title: "Marketplace" //Seen by Ultimaker as a brand name, so this doesn't get translated.
     modality: Qt.NonModal
+
+    Rectangle //Background color.
+    {
+        anchors.fill: parent
+        color: UM.Theme.getColor("main_background")
+
+        ColumnLayout
+        {
+            anchors.fill: parent
+            anchors.margins: UM.Theme.getSize("default_margin").width
+
+            spacing: UM.Theme.getSize("default_margin").height
+
+            Label //Page title.
+            {
+                Layout.preferredWidth: parent.width
+                Layout.preferredHeight: contentHeight
+
+                font: UM.Theme.getFont("large")
+                color: UM.Theme.getColor("text")
+                text: catalog.i18nc("@header", "Install Plugins")
+            }
+            Loader //Page contents.
+            {
+                Layout.preferredWidth: parent.width
+                Layout.fillHeight: true
+
+                source: "Plugins.qml"
+            }
+        }
+    }
 }

plugins/Marketplace/resources/qml/Plugins.qml

@@ -0,0 +1,9 @@
+// Copyright (c) 2021 Ultimaker B.V.
+// Cura is released under the terms of the LGPLv3 or higher.
+
+import QtQuick 2.15
+
+Rectangle
+{
+    color: "pink" //TODO
+}