Added redownload manager for previous Magnatune purchases. The required info is already stored in the released 1.4.4 version so all previously purchased albums can also be redownloaded. Added an advanced button to the magnatune browser, for now it just opens the redownload manager, but later it should popup a menu with more options. Have also done some MAJOR refactoring of the Magnatune purchasing and download logic to make it more maintanable and extensible.