Way to clean / clear Expression and Sequence list for new projects

[jenncross]

When teachers reuse the same flash drives, it would be good to have a way to easily reset the programming environment (clear out the long lists of old expressions / sequences in the palettes).

[jenncross]

This might be solved by implementing project files (instead of the current project folders) #192.

dealt with along with as per #192 in BirdBrainDev branch