add metadata to host MissionControl on Flathub

I'm investigating if we can host MissionControl on Flathub, which makes installing it really simple.

While Flathub doesn't have a policy on bundling, it is not possible to use the internet during the build. Thus I created a script (heavily inspired from here) to bundle the subprojects into a release tarball.

TODO:

• Remove unwanted files from the installation (headers, pkg-config files, etc)
• Create screenshots and add to metainfo.xml (has to be done for the guide anyway)
• Create releases entry in metainfo.xml
• Use CI to create releases and add release artifacts
• Make svg square (it is not by a tiny bit)