Performance: sprinkle 'onDestroy' everywhere to cleanup old stores; cleanup 'Stores' utility class

src/UI/Popup/LanguageElement/LanguageElement.svelte

@@ -8,6 +8,7 @@
   import type { Feature } from "geojson"
   import LayerConfig from "../../../Models/ThemeConfig/LayerConfig"
   import EditButton from "../TagRendering/EditButton.svelte"
+  import { onDestroy } from "svelte"
 
   export let key: string
   export let tags: UIEventSource<Record<string, string>>
@@ -34,7 +35,7 @@
       }
     }
     return foundLanguages
-  })
+  }, onDestroy)
 
   const forceInputMode = new UIEventSource(false)
 </script>