Demos: Work on the unsupervised P300 speller

- The spelling session will no longer restart after ending
- Changed the bandpass filter to [0.5, 12] hz
- Past letters will be remembered even after the
  max trial count of the Matlab script has been exceeded
- Changed the visualizer to use monospace font for
  the target and result texts for clarity
- Made the letters flash in bold
- Made the default target text longer