NPC shops

NPC shops work. They just have the same problem like the player/player trading dialog: no way to get any information about what you are buying. I think I will move the item details from the inventory window to an independent popup window so that it can be reused by other dialogs.

Looking at the screenshot I also notice that I mix HTML buttons with my own "link with background color and borders" buttons. I need to get rid of one of these methods to get a more consistent interface look.