Simple feature to configure alarm/reminders for an event

This patchset allows one to configure a simple alarm/reminder for an event in the calendar.
A select-box allows to choose pre-configured values for the duration. It will be embedded as an VALARM in the event-data, so that clients, which support alarms, will be able to remind you of an event, if you sync with them.

Known issues:
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* this will NOT enable owncloud to send any alarms/reminders! BUT if you sync your calendar with e.g. a smart-phone, the calendar in this client usually can handle the reminder! (At least BusinessCalendar can, and that was also my motivation to implement this feature)
* For the sake of a more easy implementation I decided to go with pre-configured values for the duration, although the format and most clients allow custom values! I used the durations that BusinessCalendar shows me when configuring an alarm, so it should be a good start.
* I also only support the alarm-action "DISPLAY", although the format also allows mail- and audio-actions
 * Also this feature only supports one alarm per event - haven't tried what happens, if you sync with a client where multiple alarms are configured