edit-dialog: don't convert date to timezone if event is all-day

Note: this is my very first code contribution to a Gnome application :)

The MR fixes #338 (closed) which is about all-day events that are shifted by one day after editing due to timezone conversions. This might be the case when, e.g., an all-day event (hour 0) is saved in local time and conversion to the best timezone (e.g., UTC) shifts the event back one day (hour 23).

I changed the code such that this timezone conversion only happens if the event is not an all-day event solving this issue. All-day events no longer move by one day after editing. Also, changing back the event from all-day to no-all-day does not seem to mess up the date.