Adding "user is typing" functionality to toxcore.

Two API functions:
```
/* Set our typing status for a friend.
 * You are responsible for turning it on or off.
 *
 * returns 0 on success.
 * returns -1 on failure.
 */
int tox_set_user_is_typing(Tox *tox, int friendnumber, uint8_t is_typing);
/* Get the typing status of a friend.
 *
 * returns 0 if friend is not typing.
 * returns -1 if friend is typing.
 */
int tox_get_is_typing(Tox *tox, int friendnumber);
```
Also added a new packet ID to messenger for the typing. Everything was done very similarly to the USERSTATUS type messages, as it's just passing a single uint8_t. So it mimics it in the various parts in Messenger as well as the api. If there are better ways to name the variables or functions, please say so. If I missed anything (as in necessary functions, anything with the callbacks, anything in init'ing or memory freeing, etc) please say so.