Store all the relevant state in a single dict (_pan_info), similar
to the one used for zoom (_zoom_info), instead of scattering it
in two attributes (_button_pressed/_xypress). Slightly reorder
logic inpress_pan to match the order inpress_zoom; likewise sync
release_pan withrelease_zoom.

We don't need to store the axes index (as was done in_xypress).

Inrelease_zoom, move the less-than-5px check out of the loop, as it
only needs to be done once.

PR Summary

PR Checklist

• Has pytest style unit tests (andpytest passes).
• IsFlake 8 compliant (runflake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs shouldbuild without error).
• Conforms to Matplotlib style conventions (installflake8-docstrings and runflake8 --docstring-convention=all).
• New features have an entry indoc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented indoc/api/next_api_changes/ (follow instructions in README.rst there).