Refactor|World|Blockmap: Blockmap now encapsulates Gridmap

The level at which Gridmap was abstracted from Blockmap is not useful.
There are however two meaningful abstractions; 1) Quadtree extraction
with floating-point space region/point lookup, and 2) external storage
allocator and accessor to the data within the Blockmap.
Merge branch 'master' of github.com:skyjake/Doomsday-Engine