Using Corrade::Utility::Directory::{read,write}() where possible.

Much shorter code now with less redundancy.
Platform: fixed assertions.
Extracted Primitive enum from Mesh to MeshPrimitive.

Because we can't forward-declare class members we would need to include
whole Mesh (along with all OpenGL headers and other stuff) just to use
Primitive enum. The old Mesh::Primitive is now alias to new one, is
marked as deprecated and will be removed in future release.
MeshTools: added TODOs.