use deletedIds map to sync deletions (#936)

* use deletedIds map for sync deletions

* refactor how we create data for syncing

* fix comments

* streamline broadcast API

* split broadcast methods

src/appState.ts

@@ -34,6 +34,7 @@ export function getDefaultAppState(): AppState {
     openMenu: null,
     lastPointerDownWith: "mouse",
     selectedElementIds: {},
+    deletedIds: {},
     collaborators: new Map(),
   };
 }