Loading and End of Timeline Delegates

Add delegate for showing the user a loading indicator and for the beginning of the timeline.

BUG: 455045
BUG: 465285

src/qml/LoadingDelegate.qml

@@ -0,0 +1,15 @@
+// SPDX-FileCopyrightText: 2023 James Graham <james.h.graham@protonmail.com>
+// SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
+
+import QtQuick
+
+import org.kde.kirigami as Kirigami
+
+import org.kde.neochat
+
+TimelineDelegate {
+    id: root
+    contentItem: Kirigami.PlaceholderMessage {
+        text: i18n("Loading…")
+    }
+}