PixelPaint: Add GuideTool for editing Guides

This patch adds the logic for the GuideTool. Pulling in from outside the
image creates a new Guide, moving a Guide outside of the image deletes
it and a Guide can be deleteted via the context menu.
A Guide is considered clicked when the cursor is less than 20 pixels
away from the line.

Userland/Applications/PixelPaint/CMakeLists.txt

@@ -17,6 +17,7 @@ set(SOURCES
     CreateNewLayerDialog.cpp
     EllipseTool.cpp
     EraseTool.cpp
+    GuideTool.cpp
     Image.cpp
     ImageEditor.cpp
     Layer.cpp