First Pass On XML-Based Store Expansions

[Dargino]

This is a first draft for expanding the silver store with new actions via companion mods.
Action data (title, description, silver cost, etc) are provided via XML, accessible via Rimworld's DefDictionary.
The definition also has a pointer to a class (inheriting from the Action abstract).
This custom action class can be located in the assemblies of any mod loaded after RimConnect (or in RimConnect's own assemblies).
A new list for all custom actions is created, and the action definitions are loaded using the GetAction() function.
The custom actions list is appended to the built-in lists to be added to the silver store.

Everything else is handled by the existing code structure.

I have tested it to the point of working without producing console errors, but rigorous bug testing has not been done.

Also, please review this carefully.. I'm not great with github, so something might be misplaced..

[Dargino]

Also, I suggest exposing the AlertManager class to public so that expansion actions can trigger the Twitch Event letters, too