Refactor|libdoomsday|FS1|Wad: Remodeled Wad internals with Entry and LumpFile

Internally Wad now uses an Entry to represent lump in the tree and a
LumpFile to represent the data payload as an FS1 compatible, File1
derived instance. The lump tree is now modeled using PathTreeT which
Wad now makes available via a const reference for efficient traversal.