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/CMakeLists.txt

@@ -31,6 +31,7 @@ set(SOURCES
     Filters/LaplaceDiagonal.cpp
     Filters/Sepia.cpp
     Filters/Sharpen.cpp
+    HistogramWidget.cpp
     IconBag.cpp
     Image.cpp
     ImageEditor.cpp