Fix Use After Free MessageDelegateBase

Make sure that a `MessageDelegateBase` is not used after free by a `MessageObjectIncubator` callback by tracking them and cleaning them up on deletion of a `MessageDelegateBase`

src/timeline/messagedelegate.h

@@ -126,6 +126,7 @@ class MessageDelegateBase : public TimelineDelegate
 
 public:
     MessageDelegateBase(QQuickItem *parent = nullptr);
+    ~MessageDelegateBase();
 
     NeochatRoomMember *author() const;
     void setAuthor(NeochatRoomMember *author);
@@ -193,6 +194,8 @@ private:
 
     bool m_isThreaded = false;
 
+    std::vector<MessageObjectIncubator *> m_activeIncubators;
+
     QPointer<QQmlComponent> m_avatarComponent;
     bool m_avatarIncubating = false;
     QPointer<QQuickItem> m_avatarItem;