Switch from Entry to TextView for multiline, paste support for attachments

src/models/message.vala

@@ -16,6 +16,17 @@ public class Message : Object
         }
     }
 
+    public bool is_attachment_marker {
+        get {
+            uint8[] attachment_marker_str = { 0xEF, 0xBF, 0xBC, 0x00 };
+            if (text.length > attachment_marker_str.length) {
+                return false;
+            }
+            
+            return (string)attachment_marker_str == text;
+        }
+    }
+
     public Message(string text, DateTime date, string? sender) {
         this.text = text;
         this.date = date;