Move remaining code to app module

There's still some stuff that could potentially go elsewhere but I think it's enough for now.

src/app/blurhashimageprovider.h

@@ -0,0 +1,26 @@
+// SPDX-FileCopyrightText: 2021 Tobias Fella <tobias.fella@kde.org>
+// SPDX-License-Identifier: LGPL-2.0-or-later
+
+#pragma once
+
+#include <QQuickImageProvider>
+
+/**
+ * @class BlurhashImageProvider
+ *
+ * A QQuickImageProvider for blurhashes.
+ *
+ * @sa QQuickImageProvider
+ */
+class BlurhashImageProvider : public QQuickImageProvider
+{
+public:
+    BlurhashImageProvider();
+
+    /**
+     * @brief Return an image for a given ID.
+     *
+     * @sa QQuickImageProvider::requestImage
+     */
+    QImage requestImage(const QString &id, QSize *size, const QSize &requestedSize) override;
+};