PixelPaint: Add a bunch of spatial filters

This patchset adds a generic convolution matrix spatial filter, and a
few named derivatives such as BoxBlur, Laplacian, Sharpen and
GaussianBlur.

Applications/PixelPaint/CMakeLists.txt

@@ -3,6 +3,12 @@ set(SOURCES
     CreateNewLayerDialog.cpp
     EllipseTool.cpp
     EraseTool.cpp
+    Filters/BoxBlurFilter.cpp
+    Filters/Filter.cpp
+    Filters/GenericConvolutionFilter.cpp
+    Filters/LaplacianFilter.cpp
+    Filters/SharpenFilter.cpp
+    Filters/SpatialGaussianBlurFilter.cpp
     Image.cpp
     ImageEditor.cpp
     Layer.cpp