UnreadBadge constructor
Displays the unread parameter in a red circle.
If not null, semanticsLabel is read instead of the unread amount if given.
maximumDigits represents how many digits will be shown. It must be equal or greater than 1 (if maximumDigits is 3, any number over 999 will be shown as 999+).
maximumDigits and unread must not be null.
Implementation
const UnreadBadge