Instead of using wglGetProcAddress directly and thus forcing a link-time
dependency on that DLL (and because this is a static lib also forcing all
users to link to it).
The function pointer loading is now moved to Application classes to make
it possible to decide about platform-specific API at usage time, not at
library compilation time.
Currently it's not possible to create the Magnum context any other way
than through Application classes, will solve that in next commits.