Added build mode options GUI:
NormalPlus option: faster building while holding RMB.
Wall and floor option: filled or hollow.
Slope floor option: raise along long edge or short edge.
Cube option: filled, hollow or skeleton.
Added quickReplaceMiningLevel to survivalBalancers in config (issue #14).
Added icons for normalSpeed, fastSpeed, full, hollow, cubeFull, cubeHollow, cubeSkeleton, shortEdge, longEdge, thickness1, thickness3, thickness5.
Fixed crash when trying to preview some modded blocks that have special rendering code. Preview will show outline instead. (issue #9 and #35)