Game modes creator

[johnklipi]

Add api which creates possibility to quickly create game modes/types via json

[IExploitableMan]

I suggest something like

namespace PolyMod;
public static class API
{
     public static void AddGamemode(string name, string description, Sprite icon, UIButtonBase.ButtonAction action)
    {
        // TODO: ...
    }
}

[IExploitableMan]

or reuse Loader class instead of new API class? @johnklipi