Implement scene specific storage for Sidebar expanded state

Multiplatform/Shared/Sidebar/SidebarExpandedContainers.swift

@@ -0,0 +1,48 @@
+//
+//  SidebarExpandedContainers.swift
+//  NetNewsWire
+//
+//  Created by Maurice Parker on 6/30/20.
+//  Copyright © 2020 Ranchero Software. All rights reserved.
+//
+
+import SwiftUI
+import Combine
+import Account
+
+final class SidebarExpandedContainers: ObservableObject {
+	
+	@Published var expandedTable = Set<ContainerIdentifier>()
+	var objectDidChange = PassthroughSubject<Void, Never>()
+	
+	var data: Data {
+		get {
+			let encoder = PropertyListEncoder()
+			encoder.outputFormat = .binary
+			return (try? encoder.encode(expandedTable)) ?? Data()
+		}
+		set {
+			let decoder = PropertyListDecoder()
+			expandedTable = (try? decoder.decode(Set<ContainerIdentifier>.self, from: newValue)) ?? Set<ContainerIdentifier>()
+		}
+	}
+	
+	subscript(_ containerID: ContainerIdentifier) -> Bool {
+		get {
+			if expandedTable.contains(containerID) {
+				return true
+			} else {
+				return false
+			}
+		}
+		set(newValue) {
+			if newValue {
+				expandedTable.insert(containerID)
+			} else {
+				expandedTable.remove(containerID)
+			}
+			objectDidChange.send()
+		}
+	}
+	
+}