Introduced a Manager to centralize plugin/package management

Should have done this from the start.
Will move other relevant scattered functions to this type.
For now it checks if the restart banner needs to show.
Taking into account that a user can toggle between enable
and disable without an actual restart. Even with multiple
plugins.

Contributes to: CURA-8587

plugins/Marketplace/resources/qml/Marketplace.qml

@@ -8,11 +8,13 @@ import QtQuick.Window 2.2
 
 import UM 1.2 as UM
 import Cura 1.6 as Cura
+import Marketplace 1.0 as Marketplace
 
 Window
 {
     id: marketplaceDialog
     property variant catalog: UM.I18nCatalog { name: "cura" }
+    property variant manager: Marketplace.Manager { }
 
     signal searchStringChanged(string new_search)
 
@@ -232,7 +234,7 @@ Window
     {
         height: quitButton.height + 2 * UM.Theme.getSize("default_margin").width
         color: UM.Theme.getColor("primary")
-        visible: CuraApplication.getPackageManager().hasPackagesToRemoveOrInstall || plugin_registry.hasPluginsEnabledOrDisabled
+        visible: manager.showRestartNotification
         anchors
         {
             left: parent.left