Extract transfer-related stuff into tox_transfer.h
Move transfer work out of main tox_thread loop
Dedent utox_thread_work_for_transfers()
Move utox_thread_work_for_transfers() to tox_transfer.h
Extract transfer callbacks setup into tox_transfer.h
Extract transfer stuff from friend.h into transfer.h
Move FILE_PROGRESS struct to transfer.h
Rename outgoing transfer functions
Make tox_transfer a compilation unit
Merge branch 'extract-transfer-code' of https://github.com/albel727/uTox