BlockType adjustments

- Flagged heads as "cannot be passed through".
- Adjusted shouldPlaceLast (Added anvils, removed heads).
- Fixed portal frame height and added a few new blocks to centralTopLimit
- Made cobblestone wall 1.5 high.
Optimized EditSession.countBlocks.

Also fixed the questionable semantics for that anonymous class' "contains" method.
Added Block and id/data overloads to canPassThrough, centralTopLimit and isNaturalTerrainBlock.
Made everything that uses canPassThrough use data values.
Made /ascend and /descend use centralTopLimit.
BlockType adjustments

- Flagged all snow layers except single layers (data values 0 and 8) as "cannot be passed through".
- Fixed centralTopLimit for trap doors and fence gates and cake blocks with data values 6..15.