Added shuffle as a helper function
Randomised presentation order and added repeat option

Repeat reading option added to config. This will present each test set
twice so that subject consistency can be checked.

The test sets are now shuffled by default. (This removes order bias as
a factor.)
Added a hidden mixture fileID

The hidden mixture fileID is required as the mixture button otherwise
highlights the hidden mixtures slider when pressed.
Bug fixes

The table is now a fixed number of rows.
The buttons for the reference and mixture need to add the audio to the
pool.
Merge with upstream