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.

src/dc_securejoin.rs

@@ -263,7 +263,7 @@ unsafe fn send_handshake_msg(
     grpid: *const libc::c_char,
 ) {
     let mut msg: *mut dc_msg_t = dc_msg_new_untyped(context);
-    (*msg).type_0 = DC_MSG_TEXT;
+    (*msg).type_0 = Viewtype::Text;
     (*msg).text = dc_mprintf(
         b"Secure-Join: %s\x00" as *const u8 as *const libc::c_char,
         step,