Because with NaCl WindowContext doesn't make sense anymore, the classes
are now renamed:
* Contexts namespace -> Platform
* *WindowContext -> Application
Hopefully this is (one of) last crazy renaming runs.
Magnum.h now doesn't include anything except OpenGL headers, thus
changes in Math library don't trigger recompilation of everything, but
only of things really depending on it.
Math constants moved to separate file for similar reasons, de-inlined
some functions to remove the need for some #includes.