With this change, kind of message is represented by value of enum
`Viewtype' instead of raw libc::c_int, providing more type safety. This
enum replaces DC_MSG_* constants. The only way to create `Viewtype' from
libc::c_int is smart constructor.
With this change, functions `dc_get_chat_media' and `dc_get_next_media' became
less forgiving about invalid message type arguments. Previously, invalid
message types were implicitly interpreted as 0 (Viewtype::Unknown). Now,
function calls with invalid message type arguments are rejected (error code
returned) on FFI boundary.
Additionally, when `Viewtype' is read from database, it is checked to have
sensible value.
No tests assumed forgiving behaviour.
pattern -- re-indentation will come after this commit with a pure application of "cargo fmt"
- bring back comment from C code
- make some path helpers return bool
