In particular, there was a typo that kept library values from being
updated, and there was missing logic to fetch the various field names
from the library parts (and the change-to part).
Also implements some performance gains by desisting from copying
LIB_FIELDs around every time we want to look at them.
Fixes https://gitlab.com/kicad/code/kicad/issues/6733
Fixes https://gitlab.com/kicad/code/kicad/issues/6749
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.