Fix toolbar separators in wx+toolmanager.

[anntzer]

The previous implementation was buggy likely because GetToolPos(tool.Id)
doesn't really work for separators, as they all share the same Id.

before:

after:

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[QuLogic]

Works in my testing. Also seems to fix the Help tool.

[anntzer]

rebase