Plugins sytem for action types

[diego-plan9]

Currently, i3 and command are the two available action types - while they are implemented via an Action trait, there are a number of places where their existence is hard-coded:

• command line enabled_action_types possible values
• conversion into specific actions at extract_action_map
• ActionType enum

It would be good to provide a small plugin-like system that can allow not only for dynamically loading specific action types, but extending seamlessly to other kinds of actions.