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/PixelPaintWindow.gml

@@ -61,6 +61,19 @@
                 max_height: 94
             }
 
+            @GUI::GroupBox {
+                title: "Histogram"
+                max_height: 90
+                layout: @GUI::VerticalBoxLayout {
+                    margins: [6]
+                }
+
+                @PixelPaint::HistogramWidget {
+                    name: "histogram_widget"
+                    max_height: 65
+                }
+            }
+
             @PixelPaint::ToolPropertiesWidget {
                 name: "tool_properties_widget"
                 max_height: 144