When loading a part in libedit, also transfer certain screen
settings to the incoming SCH_SCREEN, from the current
SCH_SCREEN.
Currently, the grid settings are transferred, but this could
also do other settings if needed.
Also remove some duplicate commentary and mention why the
grid is set in a SCH_SCREEN even though the BASE_SCREEN ctor
sets the same default.
Fixes: lp:1815108
* https://bugs.launchpad.net/kicad/+bug/1815108