Define system integration data to use for registering MIME types

Some applications are handlers for certain MIME types which might not (yet) be part of the shared-mime-info database at least on most target systems.
So for proper system integration those MIME types should be registered with the system, so that the app from the appimage can as well be registered as handler for files of those MIME types.

The spec should define what data in what files could be added to the appdir, so that system integrators (like appimaged) know what to look for and what to use.

The typical GUI linux system uses shared-mime-info. Info about deploying custom extensions (per user) can be found here: https://www.freedesktop.org/wiki/Specifications/AddingMIMETutor/

So applications which deploy custom MIME types following the XDG/FHS/shared-mime-info would normally deploy one or more files to `$PREFIX/usr/share/mime/packages` and then expect the package installer to invoke `update-mime-database` so all the new and old separate data about MIME types is integrated in the actual system database.
So for a start with the spec, given that other parts are already picking up optionally files from the XDG/FHS layout, the idea could be to point out that if there are any `*.xml` files in the AppDir at `usr/share/mime/packages`, system integration tools with XDG-based systems are recommended to deploy those files to the proper user directory (`$XDG_DATA_HOME/mime/packages`) with some unique name as also needed for undeployment, and then run `update-mime-database`.

What do you think?