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.
Bus upgrades: core new connectivity code
Bus upgrades: eeschema integration and modifications
Bus upgrades: eeschema dialogs
Bus upgrades: netlist export
Bus upgrades: file format changes
Jon Evans
Jeff Young
jean-pierre charras
Seth Hillbrand