Plugin Preferences pane 'string' field can lose cursor as it doesn't grow taller when needed #289

In Plugin Preferences pane the 'string' type gives a text field. It changes height depending on how many lines are included -- but this only happens when the panel is displayed, not when extra lines are added in a 'string' field. Therefore the cursor and new text can go out of sight quite easily. It needs to grow live as new lines are added.