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/DMSShell.qml

@@ -58,6 +58,8 @@ Item {
 
     WallpaperBackground {}
 
+    DesktopWidgetLayer {}
+
     Lock {
         id: lock
     }