Also fixed unary RectangularMatrix::operator-() and Vector::operator-()
documentation (was stating that the operation is done in-place, which is
impossible.
As there is no Magnum::TypeTraits struct anymore, there is no need to
have redundant name in it. Hopefully Doxygen will handle the difference
between this and Corrade's TypeTraits.h properly.
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.