New attachment mechanism. Also add image from clipboard.

src/matriximageprovider.h

@@ -0,0 +1,70 @@
+#ifndef MatrixImageProvider_H
+#define MatrixImageProvider_H
+#pragma once
+
+#include <QtQuick/QQuickAsyncImageProvider>
+
+#include <connection.h>
+#include <jobs/mediathumbnailjob.h>
+
+#include <QtCore/QAtomicPointer>
+#include <QtCore/QReadWriteLock>
+
+namespace QMatrixClient {
+class Connection;
+}
+
+class ThumbnailResponse : public QQuickImageResponse {
+  Q_OBJECT
+ public:
+  ThumbnailResponse(QMatrixClient::Connection* c,
+                    QString mediaId,
+                    const QSize& requestedSize);
+  ~ThumbnailResponse() override = default;
+
+ private slots:
+  void startRequest();
+  void prepareResult();
+  void doCancel();
+
+ private:
+  QMatrixClient::Connection* c;
+  const QString mediaId;
+  const QSize requestedSize;
+  const QString localFile;
+  QMatrixClient::MediaThumbnailJob* job = nullptr;
+
+  QImage image;
+  QString errorStr;
+  mutable QReadWriteLock lock;  // Guards ONLY these two members above
+
+  QQuickTextureFactory* textureFactory() const override;
+  QString errorString() const override;
+  void cancel() override;
+};
+
+class MatrixImageProvider : public QObject, public QQuickAsyncImageProvider {
+  Q_OBJECT
+  Q_PROPERTY(QMatrixClient::Connection* connection READ connection WRITE
+                 setConnection NOTIFY connectionChanged)
+ public:
+  explicit MatrixImageProvider() = default;
+
+  QQuickImageResponse* requestImageResponse(
+      const QString& id,
+      const QSize& requestedSize) override;
+
+  QMatrixClient::Connection* connection() { return m_connection; }
+  void setConnection(QMatrixClient::Connection* connection) {
+    m_connection.store(connection);
+    emit connectionChanged();
+  }
+
+ signals:
+  void connectionChanged();
+
+ private:
+  QAtomicPointer<QMatrixClient::Connection> m_connection;
+};
+
+#endif  // MatrixImageProvider_H