PixelPaint: Introduce a vectorscope

Vectorscopes are a standard tool in professional video/film color
grading. *Very* simply, the Vectorscope shows image colors with hue as
the angle and saturation as the radius; brightness for each point in the
scope is determined by the number of "color vectors" at that point. More
specifically, the Vectorscope shows a 2D UV histogram of the image,
where U and V are the chroma ("color") channels of the image.

Co-authored-by: MacDue <macdue@dueutil.tech>

Userland/Applications/PixelPaint/PixelPaintWindow.gml

@@ -77,6 +77,19 @@
                 }
             }
 
+            @GUI::GroupBox {
+                title: "Vectorscope"
+                min_height: 80
+                layout: @GUI::VerticalBoxLayout {
+                    margins: [6]
+                }
+
+                @PixelPaint::VectorscopeWidget {
+                    name: "vectorscope_widget"
+                    preferred_height: "fit"
+                }
+            }
+
             @PixelPaint::ToolPropertiesWidget {
                 name: "tool_properties_widget"
                 max_height: 144