Prevent POP3 inbox delete

app/src/main/java/eu/faircode/email/Core.java

@@ -958,6 +958,10 @@ class Core {
             message.ui_flagged = false;
         message.ui_hide = false;
 
+        // Prevent deleting message from inbox
+        if (message.uidl == null)
+            message.uidl = message.msgid;
+
         db.message().updateMessage(message);
     }