#593 Plugins for stage3

[Ben Deeming]

Implements the plugin interface for stage3. Plugins can be instantiated from within world files, and their respective interfaces from within cfg files. This patch adds external functions that plugins can call to add themselves to the already existing list of models, and implements a similar list and corresponding functions for interfaces.

[Ben Deeming]

Implements plugin interface for stage3

[Ben Deeming]

I created the patch on revision 77aaab8c35dc4df261ffb517995962797e2bbb37 from git://github.com/rtv/Stage.git