reduce ContentView redraws

Remove observability from the home model, and use inner models for
updating specific parts of the UI, such as notification dots on the tab
bar.

damus/Models/Notifications/NotificationStatusModel.swift

@@ -0,0 +1,12 @@
+//
+//  NotificationStatusModel.swift
+//  damus
+//
+//  Created by William Casarin on 2023-06-23.
+//
+
+import Foundation
+
+class NotificationStatusModel: ObservableObject {
+    @Published var new_events: NewEventsBits = NewEventsBits()
+}