-- Registry mechanism
This one's a bit more involved and I welcome comment and discussion.
Refer to eeschema/sch_io_plugin_registry.h/cpp.
A new class called SCH_IO_PLUGIN_REGISTRY is a singleton which
maintains a few indices of plugins. In order to register plugins,
each plugin class now exposes a static method called "create" which
returns the base type SCH_PLUGIN *. A pointer to this static method
is provided to one of a few helper classes which lead to index entries
created for the file extension and a string name.