Implemented Quick win: Fix the Import deck feature

One of the most common place to download decklists is from, unfortunately, their text file does not have an empty line before the word "Sideboad" and MTGO will import all the cards into the main and zero cards in sideboard.
See the tweet about the bad behavior:
Since it is a WPF app, the import code can be
private string ParseDecklist(string decklist)
return decklist.Replace("Sideboard", "\n\nSideboard");