Plugin unload/reload and Catalog toplevel/sublevel configuration added

[Ulrik Sverdrup <ulrik sverdrup gmail com>]

I have merged in a change to allow configuring exactly which sources
export their objects to the toplevel and which don't, this makes a
fourth pane in the preferences window called "Catalog". If any ideas
for needed catalog configuration that can in the same pane come up, it
would be interesting to hear.  All plugins default to exporting their
objects to the top level, to minimize surprise, but the user should
now be in full control.  There is a note about Kupfer not being
designed for a very large catalog -- does anyone have a very large
catalog / use kupfer with *a lot* of objects? What is its limits?

Also plugin "unload" has been implemented and adventurous people might
use it to even reload plugins after changing them. Lots of caveats
apply, since the plugins use the Python import mechanism, where an
import is not completely reversible. Also, memory cannot be completely
reclaimed on plugin unload; Kupfer must be restarted for this to have
its true effect.

have fun,
Ulrik