Added an option to submit items to the shop without specifying a price.
Instead of usings the tags for the shop-thread updating,
we use a new array which explicitly specify the prefix for each buyout tag.
The "No price"-tag no longer inserts a price in its row text.
The main window is now cleaned up properly upon exit.
This makes sure the buyouts are saved.
Merge pull request #37 from pajlada/master

New "No price" shop setting