Allow freely setting the % of world covered when starting a game

[asanbr]

When starting the game, there is an option for world coverage,  50% or 30% or 5% (dev), if I remember correctly.

I wanted a game where I could realistically conquer all the enemy bases within a limited time. The 5% setting has only 1 base of every faction, and this is doable, but almost too small. The 30% is way too big.

I think a 10% or 15% setting would be good for the type of game I want to play. It seems to me that allowing the player to set any % would be a very small and easy change, rather than just the fixed 5, 30, 50  choices we have now.