Resolve "Avoid windows.h pollution"

What does this MR do?

• avoid exposing of windows.h in dl::Library header (via dl::Win32.hpp)
• use abstract class dl::Native (as a pointer) in the hpp and instantiate the specific implementation in the cpp

How to test it?

• use conan packages
• build ioOdil, videoVLC and fwRenderOgre

Data

no data required

Additional tests to run

• Build ioOdil on Windows

Associated Issues

• Closes #226 (closed)