Auto-spawned units

**What do you want to see?**
In Space Empires II and III, fighters were spawned automatically by fighter bays, and despawned at the end of combat; they were not manually built and loaded into cargo. This made using them much less micromanagement intensive. The downside was that these fighters could not be designed by the player; each fighter bay component had a specific design of fighter that it carried.

We can improve on this, by doing something similar to Vector Tactics' shipyards. When you design a carrier, you will be able to select a fighter design for each fighter bay, and the same for other units, provided the fighter bays have the new "Spawn Vehicles" ability.

The ability will have various properties:
* Spawn mode - Combat (vehicles are launched in combat and are temporary), Strategic (units are spawned into the vehicle's cargo outside of combat and are permanent).
* Vehicle types - for instance, you might want your launcher to be able to launch fighters. Or maybe fighters OR mines? So a comma delimited list. Heck, why not ships?!
* Maximum tonnage - the largest hull size available to launch, or zero for no limit
* Maximum vehicles - defaults to 1, but if you set it to, say, 5, then if you choose a design that's small enough to fit five of them in the maximum tonnage then you'll get that many!
* Reload rate - how often will the units respawn? 0 for never, 1 for every round, 2 for every other round, etc. Can be a decimal if you want, same as weapons. If spawn mode is strategic this will be game turns instead of combat rounds.
* Reload mode - Fast (countdown starts as soon as units are launched), Moderate (countdown starts as soon as units are launched but new units will never launch until old ones are destroyed), Slow (countdown starts only when existing units are destroyed)

For strategically launched units, you will need to make sure there is enough cargo space on your design to store them. Combat-launched units do not have this restriction.

**How would it help players?**
Units will be much less micromanagey.

**Alternatives**
Are there any other ways these benefits could be achieved?

**Screenshots and mockups**
Please attach any screenshots of current behavior and mockups of desired behavior that help explain what you want.

**Things to consider**
What questions need to be addressed in order to implement this feature request?