Allow Jettisoning of cargo

[Kroc]

Picking up illegal cargo is annoying when you don't want to risk the offender status. There should be the ability to jettison cargo.

• [ ] Provide a key to jettison just the last thing picked up
• [ ] Allow jettisoning specific cargo from the cargo screen during flight

[Kroc]

Elite-B awards you credits for jettisoning radioactives into a star. (See also #52 for radioactives going inert)