korgac: hide "Dismiss all" button if there's only one reminder.
Summary: It's confusing to have two buttons that do the exact same thing.
Test Plan: Reminder dialog now only shows "Dismiss reminder" when there is only one reminder in the list
Reviewers: winterz, mlaurent
Reviewed By: mlaurent
Subscribers: kde-pim
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D16506