The groundwork has now been laid for per sheet instance data. Initially
this only supports sheet page numbers but could be expanded to include
other per sheet instance information.
ADDED: Support for user defined schematic page numbers.
Set up a new lineage for SCH_ITEMS to get back to the SCHEMATIC
they live on: Items will all be parented to the SCH_SCREEN that
they are added to, and each SCH_SCREEN will point back to the
SCHEMATIC that it is part of. Note that this hierarchy is not
the same as the actual schematic hierarchy, which continues to
be managed through SCH_SHEETs and SCH_SHEET_PATHS.