Add Appstream metadata

[abompard]

What does this PR do?

This PR adds the Appstream metadata file.

Motivation

An Appstream metadata file is required to show up in gnome-software, or to be listed as a GUI application in Flatpak.

Additional Notes

Reference: https://www.freedesktop.org/software/appstream/docs/

I've added the only screenshot I found on performous.org, but it's a carroussel, I'd perfer to add other higher-resolution screenshots if there are any.

[github-actions[bot]]

Download the artifacts for this pull request:

• Performous-1.2.0-913-cf8f5dd-alpha.AppImage.zip
• Performous-1.2.0-913-cf8f5dd-alpha.dmg.zip
• Performous-1.2.0-913-cf8f5dd-alpha-mingw-w64.exe.zip
• Performous-1.2.0-913-cf8f5dd-alpha-msvc.exe.zip

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[Baklap4]

I'll take a look at this when 1.3.0 is released (somewhere in the upcoming days)

[Baklap4]

Gotta take a look at itstool if it's working on windows else we might need xml_gettext.py back in