Separate button for custom quickstart

**What do you want to see?**
Add a "Custom Quickstart" button to the main menu. This will prompt the player for a custom empire, and the regular (now Default) Quickstart button should always use the default empire from the quickstart GSU.

**How would it help players?**
No more confusing prompts for quickstart empires that you have to cancel to continue with the default empire

**Alternatives**
???

**Screenshots and mockups**
N/A

**Things to consider**
What to call the buttons to minimize confusion?

Maybe add a custom dialog that loads up all the empires in the empires folder instead of just a plain old file picker dialog? Make it Blazor while I'm at it... Reuse this dialog for the load empire screen in game setup.

**Related issues**
N/A