Add AppStream Metainfo generation

[Jan200101]

Alternative to #2649

Uses the same metainfo file but fills out the release tags with git tags (when possible)
links to the git tags aren't included but are trivial to be added back

I've put the logic directly where the desktop file install is done, unsure if this is the best place to put it.

appstreamcli validate qt/io.mgba.mGBA.metainfo.xml

I: io.mgba.mGBA:6: developer-name-tag-deprecated
I: io.mgba.mGBA:~: developer-info-missing

✔ Validation was successful: infos: 2, pedantic: 1

[endrift]

I am amenable to this PR, but I think first I need a better understanding of how AppStream is used. For example, I notice GBA stuff is listed in that, but GB/C is missing. Will that pose an impediment to actually using it?

[Jan200101]

I didn't author the metainfo file but it seems to be just copied from the desktop file

the provides mediatype fields are intended to provide the same MimeType in the desktop file, if you want I can add the common mimetypes for gameboy and gbc games to both the desktop and the appstream file: application/x-gameboy-rom;application/x-gameboy-color-rom;application/x-gameboy-isx

AppStream is generally intended to be a distro independent format for providing metadata about a project, such as links to various resources, screenshots, version info, etc.

Its most commonly used in store type applications like Gnome Software or Discover and is also used by flathubs web interface

https://flathub.org/apps/io.mgba.mGBA
https://github.com/flathub/io.mgba.mGBA/blob/master/io.mgba.mGBA.metainfo.xml