Add Victoria 2 directory validator function

[Spartan322]

Validation of residing in the Victoria 2 directory should be separated from the Victoria 2 directory search so as to provide a cheaper alternative to ensure a valid Victoria 2 directory was given either through things like game settings or command line arguments, so that in the case it succeeds the search need not be performed but should it fail then we can know we must try to find it ourselves. (or request manual discovery by the user)