Color management for 16-bit images

Hello, when !79 (merged) is merged, color management for 8-bit images will work again.

Color management is not activated for 16-bit images yet. Here is 16-bit PNG: 16bitICC.zip

16-bit images use QImage::Format_RGBX64 and QImage::Format_RGBA64 formats (supported since Qt 5.12). When a high-bit image is opened by gwenview, following message is printed usually:

org.kde.kdegraphics.gwenview.lib: Gwenview can only apply color profile on RGB32 or ARGB32 images

I can investigate this further and suggest changes when ready.