The RTree does not have a deterministic iterator, so extracting items
may be in arbitrary order, causing issues as the schematic appears to
change when comparing to previous revisions.
This uses the SCH_ITEM comparison operator to for ordering by type, then
by custom sorting within type.
For the netlist, we choose the first available unit in the sheet for
each component.
Fixes#3811 | https://gitlab.com/kicad/code/kicad/issues/3811
55eb687184