Allow Planetary Movement

**What do you want to see?**
Allow planets to be moved around. Currently, planets are hardcoded to have a speed of 0 (though they can warp). By porting a similar algorithm over from the Vehicle class, replacing the Design.Hull.ThrustPerMove with something similar in the StellarObjectSize and Planet Size.txt. This should allow the planets to move around if the mod so desires, or to leave them at 0 if they don't want this.

**How would it help players?**
Would let mods add this in, if this is not added to stock FrEee.

**Alternatives**

**Screenshots and mockups**

**Things to consider**
Whether a new ability should be required or just use the Standard Ship Movement ability.
How would this affect combat? (actually, the planet running away from the troop transports sounds hilarious).
What happens if the Ringworld or sphereworld has this happen?