Calculates the longest progress bar prefix in order to keep all progress bars left-aligned.
        :return: Length of the longest task prefix in character unit.

def longest_bar_prefix_value(self):
        """
        Calculates the longest progress bar prefix in order to keep all progress bars left-aligned.
        :return: Length of the longest task prefix in character unit.
        """
        longest = 0
        for key, t in self.tasks.items():
            size = len(t.prefix)
            if size > longest:
                longest = size

        return longest