GAppInfo: add a mechanism to query supported content types

GAppInfo: add a mechanism to query supported content types

This essentially adds an accessor for the MimeType field in desktop files,
to retrieve the list of all mime types supported by an application.
The interface though is part of GAppInfo, so it could be implemented
in the future by other backends.

https://bugzilla.gnome.org/show_bug.cgi?id=674111