Compatibility with Mail Framework Mod

[Digus]

Adding a check that, when the letter was not added with FeTK, it also check is it's a LetterViewerMenu.
If it's not, it's probably a sub classe added by another mod, so it does nothing in that case.
Otherwise, it does was it's supposed to do with the vanilla mail.

This pull request also includes additions made by Pathoschild.

[dy-lin]

Error message seen in log file: https://smapi.io/log/d9fe498a25fd47f6bcb3c1e929281b38

This mod failed in the Display.MenuChanged event. Technical details: 
System.IndexOutOfRangeException: Index was outside the bounds of the array.
  at FelixDev.StardewMods.FeTK.Framework.Services.MailManager.GetMailContentForGameMail (System.String mailId) <0x12fb65f20 + 0x00134> in <filename unknown>:0 
  at FelixDev.StardewMods.FeTK.Framework.Services.MailManager.OnMenuChanged (System.Object sender, StardewModdingAPI.Events.MenuChangedEventArgs e) <0x12d0aecc0 + 0x00294> in <filename unknown>:0 
  at StardewModdingAPI.Framework.Events.ManagedEvent`1[TEventArgs].Raise (StardewModdingAPI.Framework.Events.TEventArgs args, System.Func`2 match) <0x1197258b0 + 0x00593> in <filename unknown>:0 

[Digus]

By analyzing the log and the code, it does not seem the error is specifically related to the changes added by this pull request.