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/Modules/Settings/Widgets/SettingsDivider.qml

@@ -0,0 +1,11 @@
+pragma ComponentBehavior: Bound
+
+import QtQuick
+import qs.Common
+
+Rectangle {
+    width: parent?.width ?? 0
+    height: 1
+    color: Theme.outline
+    opacity: 0.15
+}