[nemo-qml-plugin-calendar] Use correct offset for calendar reminders.

The intention is that the reminders for calendar events are
triggered before or at the start time of the event. This is done
by setting a negative start offset for the reminder.
Merge pull request #15 from pgerdt/fix-reminder-offset

[nemo-qml-plugin-calendar] Use correct offset for calendar reminders.
Merge pull request #16 from pvuorela/master

[nemo-qml-plugin-calendar] emit change signal when changing to no reminder
[nemo-qml-plugin-calendar] Avoid endless loop when editing event end time.

On handling changes, removing old events required them being
considered less than new event. Inserting new events to an index
required them being less than old event.

When editing ending time of an existing event, neither of the
conditions applied. Thus the loop didn't proceed.
Clean up whitespace