Allow to specify textfield decoration

[curiosity]

What is textfield decoration? Additional text around the value of the textfield. Most prominently visible in the train GUI and Sounds and Interface game settings.

The most practical way to do this, in my opinion, would be to let the user specify a LocalisedString. Then, when it's time to display text with decoration, one of the localised string's arguments (either first or last) gets set to the value of the textfield and the resulting localised string is displayed in place of the value.

For players it shall function exactly like the native textfield with decoration.