Add C++ clipboard image provider

Will be used to show a preview before uploading image from clipboard.
Transforming the clipboard image to PNG (or any other real format) is
slow and freezes the GUI, with the provider we can display it directly.

src/clipboard.h

@@ -82,6 +82,7 @@ public:
     }
 
 signals:
+    void contentChanged();
     void textChanged();
     void imageChanged();
     void hasImageChanged();
@@ -91,6 +92,7 @@ private:
     QClipboard *clipboard = QGuiApplication::clipboard();
 
     void mainClipboardChanged() {
+        contentChanged();
         this->hasImage() ? this->imageChanged() : this->textChanged();
         this->hasImageChanged();
     };

src/clipboard_image_provider.h

@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: LGPL-3.0-or-later
+
+#ifndef CLIPBOARD_IMAGE_PROVIDER_H
+#define CLIPBOARD_IMAGE_PROVIDER_H
+
+#include <QClipboard>
+#include <QImage>
+#include <QQuickImageProvider>
+
+
+class ClipboardImageProvider : public QQuickImageProvider {
+
+public:
+    explicit ClipboardImageProvider()
+        : QQuickImageProvider(QQuickImageProvider::Image) {}
+
+    QImage requestImage(
+        const QString &id, QSize *size, const QSize &requestSize
+    ) {
+        Q_UNUSED(id);
+
+        QImage image = this->clipboard->image();
+
+        if (size) *size = image.size();
+
+        if (requestSize.width() > 0 && requestSize.height() > 0)
+            image = image.scaled(
+                requestSize.width(), requestSize.height(), Qt::KeepAspectRatio
+            );
+
+       return image;
+    }
+
+private:
+    QClipboard *clipboard = QGuiApplication::clipboard();
+};
+
+#endif

src/main.cpp

@@ -19,6 +19,7 @@
 
 #include "utils.h"
 #include "clipboard.h"
+#include "clipboard_image_provider.h"
 
 
 void loggingHandler(
@@ -125,6 +126,11 @@ int main(int argc, char *argv[]) {
     objectContext->setContextProperty("debugMode", false);
 #endif
 
+    // Register out custom image providers.
+    // QML will be able to request an image from them by setting an
+    // `Image`'s `source` to `image://<providerId>/<id>`
+    engine.addImageProvider("clipboard", new ClipboardImageProvider);
+
     // Register our custom non-visual QObject singletons,
     // that will be importable anywhere in QML. Example:
     //     import Clipboard 0.1