Allow generic translation mechanisms

[hbons]

We should allow users to easily hook up their own translation mechanisms. In code generation, we allow to check "Make strings translatable".

This wraps all generated strings a translation method, e.g. Strings.Get("Done").

We should also generate the boilerplate class Strings.cs for the user to implement. Something like:

using MyTranslationLibrary;

class Strings
{
    public static string Get(string s)
    {
         // User implements their own translation mechanism here
         return s;
    }
}

This class will not be overwritten by updates.