[Suggestion] Secondary Invasion Timer

[TheNelston]

This is a small request to add built-in support for secondary or tertiary timers for staggered invasions. The main application would be to enable easily customizable "types" of invasions (currently, this is possible albeit with some gamestage wizardry), but I could see some additional use cases potentially arising (e.g: having an invasion which occurs over many days, with the "big" invasion itself actually being comprised of several consecutive smaller ones).