Improve quit-to-menu menu handling

This stabilizes the code that handles the menu that you land on if you
press Esc and quit to the menu.

Instead of using Game::returnmenu(), we now use the new function
Game::returntomenu() to clearly express intent that we want to return to
a specific menu. So I've added another kludge variable
Game::wasinintermission for the was-in-intermission case.

Also, I made it so that if you didn't have a main game telesave or
quicksave, you just get brought back to the main menu. Because you
shouldn't be able to go to the play menu without a quicksave or
telesave.

desktop_version/src/Game.h

@@ -236,6 +236,7 @@ public:
 
     //Menu kludge...
     bool wasintimetrial;
+    bool wasinintermission;
     bool wasincustommode;