New IClientTool interface for plugins to create Eraser-hosted tools (e.g. #147)
The Registrars are in the Eraser.Plugins.Registrars namespace
Base types are now interfaces, no longer abstract base classes
Version the Eraser.plugins assembly differently from the rest of the project so that other plugins can target different versions of Eraser with the same assembly. Change the Eraser.Plugins assembly version only if there is an API change.
Plugin loading behaviour:
Core plugins will be loaded before the rest of the plugins, but they will be loaded only by assembly name and not by path
Non-core plugin loading is handled by the Manager Library and not the Plugin code