Implemented config parameter to turn off generic legendary item drop

[FominArtmind]

Motivation: currently there is no control over generic legendary item drop. Basically, if there is only one appropriate unique not set legendary item, the chance that the legendary item drops as unique or generic one is 50/50. It's not good if someone would like to have only unique items in the game (no general legendaries at all) or possess even more granular control on legendaries drop.

Backward compatibility: doesn't effect existent users of the mod since default value for the proposed config parameter is false.

[FominArtmind]

Updated: fixed an issue with incorrect flags interaction. Basically, there is no sense in using "Set Item Drop Chance" if "Always Drop Unique Legendaries" set to true since in the latter case whether the item is set or not is better controlled by:

1. presence of set/not set corresponding items in legendaries.json
2. if multiple available items present, by their SelectionWeight parameter values ratio

[FominArtmind]

Still leads to "NullReferenceException: Object reference not set to an instance of an object
Stack trace:" when GenericLegendaryInfo added to the list. Closing for now since just don't understand C# behavior here and don't know how to fix it.

[FominArtmind]

Reworked the code to initial variant to avoid Exception.