Fix context menu not appearing in room media scroll view

MessageDelegate calls model.findEvent(root.eventId), which existed in
MessageFilterModel but was missing in MediaMessageFilterModel. This adds
a findEvent() implementation to MediaMessageFilterModel so the context
menu works correctly in the media scroll view.

src/timeline/models/mediamessagefiltermodel.cpp

@@ -87,6 +87,11 @@ QHash<int, QByteArray> MediaMessageFilterModel::roleNames() const
     return roles;
 }
 
+const Quotient::RoomEvent *MediaMessageFilterModel::findEvent(const QString &eventId) const
+{
+    return static_cast<MessageFilterModel *>(sourceModel())->findEvent(eventId);
+}
+
 int MediaMessageFilterModel::getRowForEventId(const QString &eventId) const
 {
     for (auto i = 0; i < rowCount(); i++) {

src/timeline/models/mediamessagefiltermodel.h

@@ -63,5 +63,10 @@ public:
      */
     [[nodiscard]] QHash<int, QByteArray> roleNames() const override;
 
+    /**
+     * @brief Finds the event of the given event ID in the model, returning nullptr if no matches were found.
+     */
+    Q_INVOKABLE const Quotient::RoomEvent *findEvent(const QString &eventId) const;
+
     int getRowForEventId(const QString &eventId) const;
 };