Add GitLab jobs to build and optionally publish Microsoft APPX packages

Description

These jobs port the creation of Microsoft APPX packages from Binary Factory to GitLab. Additionally, this adds the possibility to submit new versions of the app to the Microsoft Store.

Conformity

I don't think any of the below applies to this change unless there is some documentation how to publish labplot in the Microsoft Store.

• Changelog entry
• Unit tests
• Update INSTALL
• Downport
• Fixes: