Extract plugins from settings.

There isn't really any need for these two types to interact. The lua
plugin manager effectively fully owned its properties, it just delegated
all reads and writes to the settings object.

Instead, break the plugin settings out into the plugin manager and
preserve the manager in the Game. This will make it possible to expose
plugin options in the NGW without breaking the game on cancel.

game/commander/objectivefinder.py

@@ -117,7 +117,7 @@ class ObjectiveFinder:
 
                 if isinstance(
                     ground_object, IadsBuildingGroundObject
-                ) and not self.game.settings.plugin_option("skynetiads"):
+                ) and not self.game.lua_plugin_manager.is_plugin_enabled("skynetiads"):
                     # Prevent strike targets on IADS Buildings when skynet features
                     # are disabled as they do not serve any purpose
                     continue