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_location.rs

@@ -1,6 +1,7 @@
 use std::ffi::CString;
 
 use crate::constants::Event;
+use crate::constants::*;
 use crate::context::*;
 use crate::dc_array::*;
 use crate::dc_chat::*;
@@ -98,7 +99,7 @@ pub unsafe fn dc_send_locations_to_chat(
         .is_ok()
         {
             if 0 != seconds && !is_sending_locations_before {
-                msg = dc_msg_new(context, 10i32);
+                msg = dc_msg_new(context, Viewtype::Text);
                 (*msg).text = to_cstring(context.stock_system_msg(
                     StockMessage::MsgLocationEnabled,
                     "",
@@ -702,7 +703,7 @@ pub unsafe fn dc_job_do_DC_JOB_MAYBE_SEND_LOCATIONS(context: &Context, _job: *mu
                             // the easiest way to determine this, is to check for an empty message queue.
                             // (might not be 100%, however, as positions are sent combined later
                             // and dc_set_location() is typically called periodically, this is ok)
-                            let mut msg = dc_msg_new(context, 10);
+                            let mut msg = dc_msg_new(context, Viewtype::Text);
                             (*msg).hidden = 1;
                             (*msg).param.set_int(Param::Cmd, 9);
                             dc_send_msg(context, chat_id as u32, msg);