plugins/desktop-widgets: create a new "desktop" widget plugin type

- Draggable per-monitor background layer widgets
- Add basic dms version checks on plugins
- Clock: built-in clock desktop plugin
- dgop: built-in system monitor desktop plugin

quickshell/Modals/Settings/SettingsSidebar.qml

@@ -114,6 +114,12 @@ Rectangle {
                     "text": I18n.tr("System Updater"),
                     "icon": "refresh",
                     "tabIndex": 20
+                },
+                {
+                    "id": "desktop_widgets",
+                    "text": I18n.tr("Desktop Widgets"),
+                    "icon": "widgets",
+                    "tabIndex": 27
                 }
             ]
         },