Flint: Compare scans from history

Allow to compare different scans from the history.

• There was about 3 feedback about that for BLISS 1.0
• There is still feedback about that for BLISS 1.5
• PyMca provides it already (as far is i understand)
• Flint architecture was designed for
  • It's about 2 weeks for prototyping
  • 1 month to do it well

I still think it is an interesting feature to provide