Improve design persistence.

[DaloLorn]

• Whenever a design is created or updated (but not when it's loaded - bind it to the save button in the design editor!), it should be saved to a temporary design folder.
• Starting (but again, not loading) a new game should clear the folder to prevent overwriting... or perhaps the design files should be timestamped?
• A new screen or something, accessible from the main menu, should be used to select which designs to keep and under which names. (Naturally, this also removes them from the temporary folder.)
• Maybe add a new design manager tab, which uses an interface more akin to the community design list to select ships for importing into a game? (Design descriptions wouldn't be available, since those aren't actually contained in the Design object...)