PixelPaint: Add a histogram widget

This adds a simple histogram widget that visualizes the rgb-channels
and brightness for a given image. When hovering over the image it will
indicate what brightness level the pixel at the mouse position has.

Userland/Applications/PixelPaint/HistogramWidget.h

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2022, Torsten Engelmann
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "Image.h"
+#include <LibGUI/AbstractScrollableWidget.h>
+
+namespace PixelPaint {
+
+class HistogramWidget final
+    : public GUI::Frame
+    , ImageClient {
+    C_OBJECT(HistogramWidget);
+
+public:
+    virtual ~HistogramWidget() override;
+
+    void set_image(Image*);
+    void image_changed();
+    void set_color_at_mouseposition(Color);
+
+private:
+    HistogramWidget();
+
+    virtual void paint_event(GUI::PaintEvent&) override;
+
+    ErrorOr<void> rebuild_histogram_data();
+    int m_widget_height = 0;
+    Color m_color_at_mouseposition = Color::Transparent;
+    RefPtr<Image> m_image;
+
+    struct HistogramData {
+        Vector<int> red;
+        Vector<int> green;
+        Vector<int> blue;
+        Vector<int> brightness;
+    };
+    HistogramData m_data;
+};
+
+}