Last visit mode for galaxy map

**What do you want to see?**
A "last visit" mode for the galaxy map that colors star systems based on when the system was last visited

**How would it help players?**
Help players determine which systems need to be scouted

**Alternatives**
Decide on a color gradient to use. Perhaps with more recently visited systems being lighter colored, or less recently ones being redder to highlight them.

**Screenshots and mockups**
N/A

**Things to consider**
Do systems track when they were last seen/visited by an empire, or is that only space objects? If not, that will be a problem for systems with no space objects, such as nebulas and black holes.

**Related issues**
N/A