plugins: add support for composite plugins

- single plugin can register multiple types - e.g. daemon, bar widget,
  desktop widget

quickshell/PLUGINS/ExampleCompositePlugin/plugin.json

@@ -0,0 +1,21 @@
+{
+    "id": "exampleComposite",
+    "name": "Composite Example",
+    "description": "One plugin providing all three surfaces at once: a wallpaper-watcher daemon, an emoji bar widget with popout, and a desktop clock",
+    "version": "1.0.0",
+    "author": "DankMaterialShell",
+    "type": "composite",
+    "capabilities": ["daemon", "dankbar-widget", "desktop-widget", "clipboard"],
+    "icon": "extension",
+    "components": {
+        "daemon": "./CompositeDaemon.qml",
+        "widget": "./CompositeBarWidget.qml",
+        "desktop": "./CompositeDesktopWidget.qml"
+    },
+    "settings": "./CompositeSettings.qml",
+    "requires_dms": ">=1.5.0",
+    "permissions": [
+        "settings_read",
+        "settings_write"
+    ]
+}