Continuation of #966: Adding E_METAs

Adds `E_META` values to `BlockID.h`

Fixes merge issues with #966.
I think these should stay as `E_META_LEAVES_XXX`, since they are for the `E_BLOCK_LEAVES` blocktype.
Also, didn't we say we would have the NO_DECAY and CHECK_DECAY as bitwise flags, not as combined constants?