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.

src/message.rs

@@ -491,6 +491,15 @@ impl Message {
         }
     }
 
+    /// Creates a new message with Viewtype::Text.
+    pub fn new_text(text: String) -> Self {
+        Message {
+            viewtype: Viewtype::Text,
+            text,
+            ..Default::default()
+        }
+    }
+
     /// Loads message with given ID from the database.
     ///
     /// Returns an error if the message does not exist.