On the addition side, inline ToolManager._tool_added_event. It's used
at a single place, and other events emitted by ToolManager are not split
out into their own helper methods.

On the removal side, deprecate the Tool.destroy handle: it's unused, and
if a tool really wants to know when it's getting removed, it can just
connect to its own tool_removed_event, which can also be used by other
callers.

PR Summary

PR Checklist

Tests and Styling

• Has pytest style unit tests (andpytest passes).
• IsFlake 8 compliant (installflake8-docstrings and runflake8 --docstring-convention=all).

Documentation

• New features are documented, with examples if plot related.
• 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).
• Documentation is sphinx and numpydoc compliant (the docs shouldbuild without error).