[suggestion] Define Tox API in a structured form

[yurivict]

I suggest to define APIs as structures like these:

struct ApiTox {
   ...
   void *tox_callback_friend_message(Tox *tox, tox_friend_message_cb *callback);
   ...
} tox;

struct ApiToxAV {
   ...
} toxav;

struct ApiToxDNS {
   ...
} toxdns;

struct ApiToxEncryptSave {
   ...
} toxencryptsave;

This will allow to create mid-level libraries with modified APIs, for example a mid-level library augmenting Tox functionality with defragmentation, among other functions.

[yurivict]

#502