OpenSystemsDevelopment / qpdf

Move methods responsible for loading or keeping track of objects to
QPDF_objects.cc.

The split was part of the reverted #1297. Reintroducing it now makes it
easier/safer to keep the work to refactor the xref and object tables in
sync with main.

This reverts commit c648b9a018105a3c30b3e7f3c5f8a058d3ddd92c, reversing
changes made to 12b67a3227df6b6df3a4f5f098e11cce173ff7d5.

This reverts commit 54cf0e519c49e3fb44c7ca1de95a1fffbafc211c, reversing
changes made to c648b9a018105a3c30b3e7f3c5f8a058d3ddd92c.

Also, make obj_cache private and rename to table.

Move methods responsible for loading or keeping track of objects to
QPDF_objects.cc.