src/day.c: Allow editing an item's duration
src/day.c: Allow editing an item's duration

We have the option to enter either an end time or a duration when
creating an item - the same choice should be available when editing an
item.

Signed-off-by: Lukas Fleischer <af90e31281d8305af43dc616c5aaca3c7165f8a7@cryptocrack.de>

lfos/calcurse,lfos/calcurse,lfos/calcurse