Structure generation settings as in vanilla superflat

[muzikbike]

Vanilla's superflat settings have some exclusive ways to customize structure generation that vanilla custom never had the ability to customize itself. It would probably be a good idea to port such options over to FlatCubic and CustomCubic as well for further customization capabilities, with the options potentially more adapted to a cubic world where appropiate. Non-functional sliders and checkboxes could be added for these almost immediately, as they'd be covered by the fix to #91 anyway.

The following options exist in vanilla superflat 1.15:

• Village size
• Village distance
• Mineshaft chance
• Stronghold distance
• Stronghold count
• Stronghold spread
• biome_1 (desert temple/jungle temple/witch hut/igloo - should probably be separated) distance
• Woodland mansion distance
• Ocean monument spacing
• Ocean monument separation
• End city distance