PixelPaint: Add ProjectLoader to abstract away opening of files

This new class will open and parse files (either images directly or .pp
project files) and one can get the parsed Image as well as other
information from it.

This patch removes a bunch of 'try_create_from..." methods from Image in
favor of using the ProjectLoader.

The only json_metadata that is available are Guides for now.

Userland/Applications/PixelPaint/CMakeLists.txt

@@ -30,6 +30,7 @@ set(SOURCES
     PenTool.cpp
     PickerTool.cpp
     PixelPaintWindowGML.h
+    ProjectLoader.cpp
     RectangleTool.cpp
     RectangleSelectTool.cpp
     Mask.cpp