1) Zero out selection in tree when canvas captures focus so that
save/save as actions will go to canvas, not tree item.
2) Make save do a save to schematic when the canvas symbol is from
the schematic.
3) Copy fields when doing a save to schematic (setting the lib part
doesn't update them on its own).
4) Remove no-longer-necessary calls to update schematic after
saving symbols (the schematic symbols are no longer hot-linked).
Fixes https://gitlab.com/kicad/code/kicad/issues/6763
Adds Cut/Copy/Paste and Revert for footprints; introduces a new
shared Revert Changes? dialog; hooks up Add Library for footprints,
standardizes the Save As terminology.
Jeff Young