MKOB: Support DELAY prosign in keyboard sender

[AESilky]

For training/learning, it would be helpful if the keyboard sender supported a prosign to add a delay.
For example, [d2] would add a 2-dot delay at the current speed into the string.

Like: "Hello there[d2]"
When used with the Repeat option, would add a two dot delay after sending the last 'e'.