Do avatar placeholder in SwiftUI, Add AvatarViewModel, Add dependency injection for MxMediaManager dependency.

Riot/Modules/Common/SwiftUI/VectorHostingController.swift

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+// 
+// Copyright 2021 New Vector Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+import Foundation
+import SwiftUI
+
+/**
+ UIHostingController that applies some level-app specific configuration
+ (E.g. vectorContent modifier and themeing to the NavigationController container.
+ */
+@available(iOS 14.0, *)
+class VectorHostingController: UIHostingController<AnyView> {
+    
+    // MARK: Private
+    
+    private var theme: Theme
+    private var dependencyContainer = DependencyContainer()
+    
+    init<Content>(rootView: Content) where Content: View {
+        self.theme = ThemeService.shared().theme
+        
+        super.init(rootView: AnyView(rootView.vectorContent()))
+    }
+    
+    required init?(coder aDecoder: NSCoder) {
+        fatalError("VectorHostingViewController does not currently support init from nibs")
+    }
+    
+    // MARK: - Life cycle
+    
+    override func viewDidLoad() {
+        super.viewDidLoad()
+        self.registerThemeServiceDidChangeThemeNotification()
+        self.update(theme: self.theme)
+    }
+    
+    func add<T>(dependency: T) {
+        dependencyContainer.register(dependency: dependency)
+    }
+    
+    private func registerThemeServiceDidChangeThemeNotification() {
+        NotificationCenter.default.addObserver(self, selector: #selector(themeDidChange), name: .themeServiceDidChangeTheme, object: nil)
+    }
+    
+    @objc private func themeDidChange() {
+        self.update(theme: ThemeService.shared().theme)
+    }
+    
+    private func update(theme: Theme) {
+        self.view.backgroundColor = theme.headerBackgroundColor
+        
+        if let navigationBar = self.navigationController?.navigationBar {
+            theme.applyStyle(onNavigationBar: navigationBar)
+        }
+    }
+}