Implement the save action when previewing media. (#3630)

* Implement the save action on the media preview.

* Update Compound and use the correct icon.

Also fixes an icon that has been renamed.

* Update the add to photo library usage description to match the designs.

* PR comments.

ElementX/Sources/Screens/FilePreviewScreen/View/TimelineMediaPreviewFileExportPicker.swift

@@ -0,0 +1,23 @@
+//
+// Copyright 2024 New Vector Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only
+// Please see LICENSE in the repository root for full details.
+//
+
+import SwiftUI
+
+struct TimelineMediaPreviewFileExportPicker: UIViewControllerRepresentable {
+    struct File: Identifiable {
+        let url: URL
+        var id: String { url.absoluteString }
+    }
+    
+    let file: File
+    
+    func makeUIViewController(context: Context) -> UIDocumentPickerViewController {
+        UIDocumentPickerViewController(forExporting: [file.url], asCopy: true)
+    }
+    
+    func updateUIViewController(_ uiViewController: UIDocumentPickerViewController, context: Context) { }
+}