Add delegates to show room upgrades into the timeline model.

The delegates are at the beginning for upgraded rooms and end for predecessors.

Closes: network/neochat#620 and network/neochat#619

src/timeline/SuccessorDelegate.qml

@@ -0,0 +1,33 @@
+// SPDX-FileCopyrightText: 2024 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 QtQuick.Layouts
+
+import org.kde.kirigami as Kirigami
+
+import org.kde.neochat
+
+TimelineDelegate {
+    id: root
+
+    /**
+     * @brief The current room that user is viewing.
+     */
+    required property NeoChatRoom room
+
+    width: parent?.width
+    rightPadding: NeoChatConfig.compactLayout && root.ListView.view.width >= Kirigami.Units.gridUnit * 20 ? Kirigami.Units.gridUnit * 2 + Kirigami.Units.largeSpacing : Kirigami.Units.largeSpacing
+
+    alwaysFillWidth: NeoChatConfig.compactLayout
+
+    contentItem: Kirigami.InlineMessage {
+        visible: true
+        text: i18n("This room has been replaced.")
+        type: Kirigami.MessageType.Information
+        actions: Kirigami.Action {
+            text: i18n("See new room…")
+            onTriggered: RoomManager.resolveResource(root.room.successorId)
+        }
+    }
+}