Pavlovia compatibility

[MikoBie]

This adds pavlovia compatibility but also fixes a bug that occurred when one wanted to delete a letter in the recall segment. It also fixes some minor inconveniences by adding the percentage sign and correcting how the percentages are computed. The major change is though on how the letters are displayed. Before all letters to be displayed were drawn at the beginning of the loop, now a single letter is drawn with each iteration. Although it is a technicality it has grave consequences:

1. The first routine of displaying letters always starts with the letter 'z'. Although it is a bit peculiar solution it does not affect the task itself because it does not matter whether the first letter is set if others are drawn randomly, for example in the first memory task the first routine would always consist of 'Z' while the second and every next routine would have random letters as one would expect.
2. The same letter can be drawn multiple times in the same routine. Although it is rather unlikely to have the same letter drawn more than twice. For example, the same letter could be drawn consequently but it is rather unlikely.

[zen-juen]

@MikoBie Hi there, thanks so much for this, I'm currently swamped with deadlines and exams at the moment and will get back to this as soon as I can (should be ~3 weeks from now)