...to avoid confusion.

Use a customized QPlainTextEdit instead of a QLineEdit because the
latter does not allow wrapping of long lines. Horizontal scrolling in a
QLineEdit for long/many options is inconvenient.

Change-Id: I15b5f034e04920d0ca955c4f822d882b05e9c235
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>