Blender Git Loki

Added two RNA struct functions - merely wrappers around the C api.

- add_mesh to Main - calls C add_mesh and returns a new mesh
- copy to Mesh - calls C copy_mesh and returns a new copy

Not sure about function placement and naming though.
Put both functions in editmesh.c, mesh editor module. Added prototypes
to rna_internal.h. Prefixed both with "RNA_api_".

Wanted to code Mesh.copy so that it copies Mesh data from another object
instead of creating a new Mesh, but this needs CustomData manipulations
which I should study later.

Maybe we need a separate file for API functions? e.g. mesh_api.c?