New "No price" shop setting

The option previously called "No price" has been renamed to "Ignore", and a new option takes the name "No price".
Any tab or item set as "No price" will be included in the shop but have no price set.

The buyout data was not always saved when only modifying tab buyouts, so the login dialog is now responsible for making sure the main window is cleaned up properly.
I don't see this causing any problems, as the Login dialog is only cleaned up on application exit anyway, which should be around the time the main window should be cleaned up anyway.