Confirmation dialog when starting a game

**What do you want to see?**
When starting a game from game setup, there should be a confirmation dialog when clicking the start button.

**How would it help players?**
Prevent players from accidentally starting games prematurely (e.g. when trying to save their empire file)

**Alternatives**
???

**Screenshots and mockups**
N/A

**Things to consider**
What should be displayed in the dialog besides yes/no? Maybe the game name, number of human/AI players, map size, number of homeworlds?

**Related issues**
N/A