Dedicated artists for inset colorbars, centered-row legends

[lukelbd]

Centered-row legends and inset colorbars are currently implemented on the Figure and Axes methods with colorbar_wrapper and legend_wrapper. But this means background patch tasks that would be carried out by the Colorbar and Legend classes at draw-time have to be done during call-time.

• For legend, this may cause hard-to-diagnose display errors if the renderer being used to position the "centered-legend" box changes between legend() call-time and draw-time.
• For colorbar, we worked around this by severely restricting the bounding box options and estimating the necessary space from font sizes. But it would be much better if the box shape was adjusted dynamically to encompass colorbar contents.

This PR will create dedicated classes for inset colorbars and centered-row legends so the background patch extent can be determined dynamically at draw-time.

Also, while starting this PR I learned about some intriguing matplotlib classes in the offsetbox module used to automatically position legend labels / text. The HPacker and VPacker classes might be used to implement #46 / #50. Anchored artists might also be useful for both PRs.

[lukelbd]

This branch is too out of date to merge. But it is the next item on the to-do list so will be implemented before long (by the end of the year).