QLineEdit::inputMask: Add mask characters to allow only ASCII

QLineEdit::inputMask was documented until Qt 5.14.1 to limit the input for A/a/N/n/9/0 mask characters to the ASCII range, when in fact it supported any unicode character from the Letter or Number category, respectively. See QTBUG-82291.

Limiting input to the ASCII range is nevertheless useful. While this can be implemented using a QRegularExpressionValidator, direct support through the inputMask could still be useful.

This could be achieved with a meta-character that defines that all following characters need to be in the ASCII range; we could use the already reserved bracket characters for this.

Or it could be new dedicated mask characters, in which case we would need several characters, e.g ASCII equivalents for A, a, N, n, 9, 0, D, d, and #.