Map API: BspLeaf replaced with ConvexSubspace

ConvexSubspace replaces BspLeaf as the public representation for a
convex subspace in the BSP tree. BspLeaf and BspNode are no longer
addressable via the Map API (DMU).

ConvexSubspaces are now owned by de::map rather than BspLeaf, and
no longer provides public access to the old BSP map element LUTs
(which are now only used to ensure allocations are released when
the map is unloaded).

The Map API version was bumped to ver 4.