Start moving towards saner RA widget logic

- Let the shellmaps open the main menu. This lets us not hardcode MAINMENU_BG everywhere.
- Unshellmapify all the D2k MP maps and have a real shellmap. This shellmap doesn't actually do anything but it doesn't make the current D2k shellmap situation worse.
- Swaps to the observer widgets after gameover so you have access to the shroud selector, observer radar and player stats. Fixes #2522 and removes massive duplication
- This also lets us remove the legacy OpenWidgetAtGameStart trait in favour of LoadWidgetAtGameStart.