The plugin objects are MarshalByRefObject stubs, which means they
don't actually implement the interfaces we're checking for. There's
some additional overhead to do the interface check. We can avoid
it by doing the interface queries during initialization, and just
checking some bit flags later on.
Also, in the extension script info window, show a list of
implemented interfaces.
Once upon a time, symbol files and extension scripts could only be
defined in the RuntimeData directory, so having the documentation
there made sense. Since both of these things can now be defined in
project directories, the documentation belongs in the manual.
(issue #27)
