Allow simultaneous visualization of several skies

In the teachers' workshop at ESTEC it was suggested that a very useful feature would be the possibility to visualize several skies/sky regions at the same time. It is something that had already been suggested before. This would make comparisons easier between different objects in the same sky, or between observations of the same object by different missions or in different passbands. 

I can think of two possibilities: via tiling (dividing the window into smaller pieces) and via transparency. Both are implemented in Aladin Desktop. Maybe we should think of possible issues with the data panel interaction, though...