The legacy and s-expression (prior to version 20200828) file formats only
supported symbol unit and reference fields so the newly added value and
footprint fields must be updated from the original symbol fields.
Fixes https://gitlab.com/kicad/code/kicad/-/issues/12226
1) Generalize SCH_ITEM owners (SCH_COMPONENT, SCH_SHEET, and now
SCH_GLOBALLABEL)
2) Generalize hypertext items
3) Use SCH_FIELD autoplace infrastructure for placing intersheet
references
4) Use textVar infrastructure for buildin intersheet references.
As an important side-effect this also fixes the undo issues with
intersheet refs.
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.
Jeff Young
Roberto Fernandez Bautista
Wayne Stambaugh
Sylwester Kocjan
Dominik Wernberger
Marek Roszko
Jon Evans