This branch is the start of work to unify the storage of the artists in the Axes into a single tree. I think this will simplify the serialization and export code we want to add. I also think this will make it easier to implement logic to check if a canvas needs to re-drawn. In all cases, the benefit comes from having a single Artist tree to walk.

The first step of this is to promoteContainers to be full-classArtists and use them for drawing (not just sorting out legend handles and returning). This will fix the z-order overlap issues with error bar (#409).

There is some magic in how the broadcasting of the{set,get}_* methods works which should probably have some discussion around it.

The sub-class oflist is so that we can maintain back-compatibility for users that are removing artists by mucking with the internal data structures.

• intra-container zorder
• update all usage ofContainer sub-classes (only ~5)
• unify draw logic to a single tree
• make sure all the back compatibility works
• make remove fully clean up afteradd_*
• deprecate all of theadd_* methods
• add visitor-pattern API