RFC: Plugin framework design

Here's a first draft of the plugin framework design:

https://gitweb.torproject.org/chiiph/vidalia.git/blob/ade1882477472b871975e70d18dce87cf911eb55:/doc/plugin-framework.txt

The development will take place at my branch chiiph/plugin.

I thought about documenting examples for each of the parts that need to be developed, but I think it's better to have them when we know code like that is working properly with the design I'm proposing.

Thoughts?