Deduplicate enums and typedefs from tox.h and lower layers

[Ansa89]

@iphydf wrote:

problem: the tox public interface defines a lot of enumerations that are duplicated in the implementation

Proposed solutions:

• let directly include tox.h (carefully checking that only enums/types are used from there)
• split tox.h into two files: one for enums/types, one for functions