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.
Replace SEVERITY_ERROR by RPT_SEVERITY_ERROR to avoid collision with a system definition.
Replace other SEVERITY_XXX by RPT_SEVERITY_XX for consistency.
Fix compil warnings and some other compil issues.
ADDED: Back annotation algorithm,
eeschema back annotation dialog
CHANGED: added some minor helper methods to SCH_REFERENCE_LIST and SCH_REFERENCE,
split SCH_REFERENCE_LIST::CheckAnnotation on 2 parts to reuse code
