Empire picker dialog

**What do you want to see?**
An empire picker dialog which loads up all the empire files in the Empires folder as if they were being loaded into a game, and displays their name, flag, empire points spent, etc. This dialog can be used for loading empires into a game and also picking the empire for a quickstart game.

**How would it help players?**
Easier to navigate available empires?

**Alternatives**
???

**Screenshots and mockups**
N/A

**Things to consider**
What if players want the filename because they're loading empires for multiplayer? Should display that in the list as well.

**Related issues**
#351 - custom quickstart button that prompted this idea