Split 'Image.hpp' into three submodules.
Rename 'Color.hpp' as 'Pixel.hpp'
Fix 'Core.hpp'
Move 'Subimage.hpp' to subfolder 'Image'.
Move 'Subimage.hpp' to subfolder 'Image'.
Remove redundant assignment operator.
Add stubs unit tests for the image module.
Add unit tests for utility functions.
Clean up the `ConvertImage` class.
Add and fix 'convert_channel' and 'convert_color' functions.
Add `convert_color` and `convert_channel` methods to `Image` class.

The API is much more cumbersome but necessary in my opinion. We will see
later how to improve the API.
Add missing header inclusion.
Remove template function 'loadFromDialogBox'.
Fix unit tests for color rescale.
Prevent color rescaling for image with integral channels.
Fix typos.
Merge pull request #109 from DO-CV/maint-divide-image-hpp

[MAINT] Divide `Image.hpp` into several header files.