fix(services/spotlight): improve MPRIS artwork handling and scope keyword searches to single plugin

- mpris: native MediaArt cache lookup, dedupe play/pause emission, safe
  cleanup, artwork polling for already-playing browser players, drop
  os.system seek fallback
- spotlight: scope plugin keyword and detect() matches to a single plugin
  via search_single instead of global search; register clipboard keywords
  (clip/clipboard)

src/shared/window/animated_scrollwindow.py

@@ -80,3 +80,7 @@ class AnimatedScrollable(ScrolledWindow):
         value = self.height_animator.value
         value = 0 if value < 0 else value
         return value, value
+
+    def destroy(self):
+        self.height_animator.stop()
+        super().destroy()