Refactor Plugin Code and Add ability to have pre/post execution plugins

[ThinkingEngine-net]

• Centralise plugin code
• Add a new plugin class type that supports pre and post-execution plugins (e.g. to integrate with test management systems)
• Add the ability to create config screens for plugins.
• Add the ability to specify a custom plugin path

[ThinkingEngine-net]

PLugin code centralised and custom path configurable in preferences

[ThinkingEngine-net]

Done