271|Refactor: Stax keymap as file

Description of refactor

• Stax's keymap is defined by a file stax_keymap.py and loaded at startup.
• userpref.blend is cleaned from any Stax keymap and default "Blender" keymap is restored to default.

Closes #271 (closed)

How to test it

Use Stax as usual.

Checklist

Please check if your MR fulfills the following requirements:

• Unit tests have been adapted
• Build was run locally and any changes were pushed
• Lint has passed locally without error
• CHANGELOG.md has been updated

FYI: @awesome_dev_too