Use proxy objects on lambdas instead of raw pointers

When the object is destroyed the connections will be removed
automatically by Qt.

fixes #433

src/MatrixClient.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <QMetaType>
+#include <QObject>
 #include <QString>
 
 #include <mtx/responses.hpp>
@@ -17,6 +18,16 @@ Q_DECLARE_METATYPE(std::string)
 Q_DECLARE_METATYPE(std::vector<std::string>)
 Q_DECLARE_METATYPE(std::vector<QString>)
 
+class MediaProxy : public QObject
+{
+        Q_OBJECT
+
+signals:
+        void imageDownloaded(const QPixmap &);
+        void imageSaved(const QString &, const QByteArray &);
+        void fileDownloaded(const QByteArray &);
+};
+
 namespace http {
 mtx::http::Client *
 client();