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.
Short net names are not unique; full names with paths must be used.
Added a grid cell renderer that does the escaping, to make it easier
to display net names in grid cells.
Once you unescape a net name, you can't go back to the escaped form
because you can't assume which `/` should be {slash} and which `/`
Because of this, we cannot use Unescape/Escape on the data model in
the netclass setup panel, and instead do the Unescape in the view.
Fixes https://gitlab.com/kicad/code/kicad/-/issues/5331
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
Wayne Stambaugh
Jon Evans