Add properties to the retro.info files

[hadess]

I'd be happy to get the ball rolling on this, but want to make sure that the changes would be accepted.

We'd like to have supported_mimetypes and supports_serialization metadata. supported_mimetypes would be the supported mime-types as defined by shared-mime-info. supports_serialization would include whether the core can save state.

We could then use the same .info files in gnome-games, and share the burden of maintaining them, rather than have gnome-games hard-code supported cores.

[inactive123]

If it is just about adding those to the core info files, that is fine with me.

[hadess]

It would be, yes. Either @Kekun or I will look into a first pass at it.