PR Summary

This enables the toolbar buttons to work with 3D axes now. The panning works pretty seamlessly, but the zooming is perhaps not the easiest to use still. This is because it is zooming based on relative locations between the selected box and the Axes, but the Axes actually expands beyond the corners seen in the 3D grid world. If you zoom using the x/y keys to constrain the axes, you will see that the zoom box actually goes a ways outside the 3D world, and then behaves quite well when constrained like that from an intuitive sense. I'm open to suggestions here if anyone can think of a better way to implement the zoom logic, or if we think it is still a bit too complicated and that tool should not be implemented at all.

Test with any of the 3D examples:
https://matplotlib.org/stable/gallery/mplot3d/surface3d.html
https://matplotlib.org/stable/gallery/mplot3d/scatter3d.html

Commit summary:

This moves the pan logic that was already in the mouse move handler
into the "drag_pan" method to make it available from the toolbar.

This also expands upon the panning logic to enable a zoom-to-box feature.
The zoom-to-box is done relative to the Axes, so it shrinks/expands
the box as a fraction of each delta, from lower-left Axes to lower-left
zoom-box. Thus, it tries to handle non-centered zooms, which adds more
cases to handle versus the current right-click zoom only scaling from
the center of the projection.

Closes #110

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• New features are documented, with examples if plot related.
• [] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [N/A] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).