api: Add Message::new_text() (#6123)

This adds a function to `Message`:

```rust
    pub fn new_text(text: String) -> Self {
        Message {
            viewtype: Viewtype::Text,
            text,
            ..Default::default()
        }
    }
```

I keep expecting that a function like this must exist and being
surprised that it doesn't.

Open question is whether it should be `pub` or `pub(crate)` - I made it
`pub` for now because it may be useful for others and we currently we
aren't thinking about the Rust API that much, anyway, but I can make it
`pub(crate)`, too (then it can't be used in deltachat-jsonrpc and
deltachat-repl).

I replaced some usages of Message::new(Viewtype::Text), but not all yet,
I'm going to do this in a follow-up, which will remove another around 65
LOC.

deltachat-repl/src/cmdline.rs

@@ -1004,8 +1004,7 @@ pub async fn cmdline(context: Context, line: &str, chat_id: &mut ChatId) -> Resu
             ensure!(sel_chat.is_some(), "No chat selected.");
 
             if !arg1.is_empty() {
-                let mut draft = Message::new(Viewtype::Text);
-                draft.set_text(arg1.to_string());
+                let mut draft = Message::new_text(arg1.to_string());
                 sel_chat
                     .as_ref()
                     .unwrap()
@@ -1028,8 +1027,7 @@ pub async fn cmdline(context: Context, line: &str, chat_id: &mut ChatId) -> Resu
                 !arg1.is_empty(),
                 "Please specify text to add as device message."
             );
-            let mut msg = Message::new(Viewtype::Text);
-            msg.set_text(arg1.to_string());
+            let mut msg = Message::new_text(arg1.to_string());
             chat::add_device_msg(&context, None, Some(&mut msg)).await?;
         }
         "listmedia" => {