Problem

Recently I had a case of layouting subfigures where I could have benefitted from a subfigure_mosaic function, similar to subplot_mosaic.

Proposed solution

Provide subfigure_mosaic.
Bonus points if the implementation shares as much as possible with subplot_mosaic (I suspect we can just factor out everything and pass the "child-adding method" (fig.add_subplot or fig.add_subfigure) as a parameter to the internal implementation?).