Reworked main menu, new title artwork and credits

The main menu, the splash screen and the loading screen show updated title artwork. The menu itself has been reorganised.

New

• Credits: In the bottom right corner of the main menu, "Credits" opens a window listing everyone who helped with the playtests.

Improved

• Title artwork: The splash screen, the loading screen and the main menu show the new version of the title image.
• Structure: "Continue" stands apart at the top, followed by "New game", "Join a game", "Settings" and "Achievements", with "Quit game" (formerly "Quit") set apart at the bottom.
• Look: The background image is barely darkened any more, and the buttons fill the full width of the menu.
• Achievements: A "Back" button in the top left corner closes the window, also when it was opened from the main menu.