Introduce new enum: Viewtype

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.

deltachat-ffi/Cargo.toml

@@ -18,6 +18,7 @@ crate-type = ["cdylib", "staticlib"]
 deltachat = { path = "../", default-features = false }
 libc = "0.2"
 human-panic = "1.0.1"
+num-traits = "0.2.6"
 
 [features]
 default = ["vendored", "nightly", "ringbuf"]