Ship/fleet/unit experience

**What do you want to see?**
SE4 let ships and fleets gain experience which gave them bonuses to accuracy and evasion.

**How would it help players?**
It feels cool to have an experienced "hero" ship?

**Alternatives**
I want to make it more like SE5: experience is made of points that cause your ship or fleet to level up and gain new abilities. These levels and abilities will be defined in a mod file. Also maybe allow experience for units. Maybe get rid of fleet experience or rework it somehow because it's kinda cheesy the way it works currently... Ooh, what if we add experience for colonies? Experience levels can also have rank icons that get attached to the appropriate unit so you can see at a glance which fleets are experienced.