Add AppStream

[JakobDev]

This PR adds a AppStream file for Linux. AppStream contains the data for Software centers on Linux (e.g.g Gnome Software or KDE Discover) like Description, Screenshots, License, Links etc. If a App provides no AppStream file, the data from the .desktop files are used, which is missing a lot of things of course.

This AppStream is currently missing some screenshots, as I can't find some good. Please provide the Link to at least one Screenshot, so I can add them before merging.

[endrift]

Is this meant to be distributed along with the application? There's no way I'm going to remember to update the release list every single time I make a new release...there are multiple versions of mGBA that just straight up show the wrong release number in places since I forgot to update it for a minor release.

[JakobDev]

Is this meant to be distributed along with the application?

Yes

There's no way I'm going to remember to update the release list every single time I make a new release.

Maybe this can be automated with GitHub actions are this can be set during the build with CMake.

[Jan200101]

Hey, is there any plan to continue this PR?
I've implemented some cmake logic that automatically populates the releases list through the git cli, though it doesn't include a type or the tag url
Jan200101@2cedad1