Reminders are triggered after the event instead of before

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Description

### To Reproduce
Steps to reproduce the behavior:
1. Open the calendar
2. Create an event with a reminder 15 minutes before the event
3. Open evolution
4. Go to the properties of the event
5. Evolution shows that the event has a reminder 15 minutes *after* the event

### Expected behavior
5. Evolution shows that the event has a reminder 15 minutes before the event

### Additional information
The same behavior happens if you sync the calendar via caldav and view it with the android application etar.

An example event created in elementary-calendar with a 15 minute reminder before the start:
```
BEGIN:VCALENDAR
VERSION:2.0
BEGIN:VEVENT
UID:218e683cb8bd8d7efeb9fa35bb07c408af01d909
DTSTAMP:20191017T181542Z
SUMMARY:test
DTSTART;TZID=/freeassociation.sourceforge.net/Europe/Paris:
20191016T201542
DTEND;TZID=/freeassociation.sourceforge.net/Europe/Paris:20191016T211542
X-LIC-ERROR;X-LIC-ERRORTYPE=VALUE-PARSE-ERROR:No value for DESCRIPTION
property. Removing entire property:
X-LIC-ERROR;X-LIC-ERRORTYPE=VALUE-PARSE-ERROR:No value for LOCATION
property. Removing entire property:
CREATED:20191017T181557Z
LAST-MODIFIED:20191017T181557Z
BEGIN:VALARM
X-EVOLUTION-ALARM-UID:b51c233c432295b835a0c71d14bde180d13f8bdb
ACTION:DISPLAY
TRIGGER;RELATED=START:PT15M
END:VALARM
END:VEVENT
END:VCALENDAR
```
According to the specification, it should have been: `-PT15`
see https://icalendar.org/iCalendar-RFC-5545/3-8-6-3-trigger.html

## Platform Information
- OS: NixOS 19.09.741.dbad7c7d59f (Loris)
- [ ] I'm using the latest version from git that I've manually compiled
- [x] I'm using the latest released stable version (5.0)

---
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