These changes are motivated by feedback from Tim Hunt (17 October 2011
in the developers chat room). The combination of using an interface, but
then putting all the useful information in public fields, was not good
design of this library. So I am merging the plugin_information interface
and the plugintype_base class into a new abstract class plugininfo_base.
This new class defines the basic API of all information holding
subclasses. This does not change the code, just puts the documentation
closer to the methods they are actually implemented.
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