Split message content into its own module

This is laying some groundwork for the rich text chatbar.

src/messagecontent/ReplyButtonComponent.qml

@@ -0,0 +1,42 @@
+// 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 QtQuick.Controls as QQC2
+import QtQuick.Layouts
+
+import org.kde.kirigami as Kirigami
+import org.kde.kirigamiaddons.delegates as Delegates
+
+import org.kde.neochat
+import org.kde.neochat.chatbar
+
+/**
+ * @brief A component to show a reply button for threads in a message bubble.
+ */
+Delegates.RoundedItemDelegate {
+    id: root
+
+    /**
+     * @brief The thread root ID.
+     */
+    required property string threadRoot
+
+    Layout.fillWidth: true
+    Layout.maximumWidth: Message.maxContentWidth
+
+    leftInset: 0
+    rightInset: 0
+
+    highlighted: true
+
+    icon.name: "mail-reply-custom"
+    text: i18nc("@action:button", "Reply")
+
+    onClicked: {
+        Message.room.threadCache.replyId = "";
+        Message.room.threadCache.threadId = root.threadRoot;
+        Message.room.mainCache.clearRelations();
+        Message.room.editCache.clearRelations();
+    }
+}