Separate button for custom quickstart

[ekolis]

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