Infrastructure: improve Windows builds [skip actions]

[SlySven]

Brief overview of PR changes/additions

First off attempt to investigate AppVeyor build environment.

Motivation for adding to Mudlet

We need to try and reduce the dependency on the upstream mingw site which has become somewhat unreliable in the last few weeks.

Other info (issues closed, discussion etc)

[add-deployment-links[bot]]

Hey there! Thanks for helping Mudlet improve. :star2:

Test versions

You can directly test the changes here:

• linux: https://make.mudlet.org/snapshots/590979/Mudlet-4.17.2-testing-pr7207-02313949-linux-x64.AppImage.tar
• osx: https://make.mudlet.org/snapshots/99a4e7/Mudlet-4.17.2-testing-pr7207-62a9180d.dmg
• windows: (download pending, check back soon!)

No need to install anything - just unzip and run. Let us know if it works well, and if it doesn't, please give details.

[SlySven]

I'm probably going to close this PR in favour of a later one.