Updated toolchains submodule.
Indent the nested ifdefs for better readability.
Move extension loading into Magnum::Context.

Resolves some linker errors in Windows, removes code bloat from Platform
namespace (previously it was copied in _every_ *Application).

Originally this was in Platform namespace because I thought that it
would be possible to have one version of Magnum library for both desktop
and ES and the actual Application would determine which GL edition will
be used. Fun idea, but it would remove the static checks (e.g.
accidentaly using geometry shaders on ES), which isn't worth it.