Summary:
Or in layman terms, don't pop up reminders during a presentation.

Plasma has a "do not disturb" setting (which is even automatically
enabled when mirroring the screen onto a projector), and that setting is
available via a property in a freedesktop notifications interface.

The code is in alarmdialog rather than alarmclient because the "suspend"
code is in alarmdialog and calls wakeUp() directly.

Test Plan:
- Create event in korganizer with reminder in 2 minutes
- Enable "do not disturb" mode in Plasma (Notifications systray icon)
- Wait for > 2 minutes, no reminder
- Disable "do not disturb" mode
- The reminder shows immediately

- Click Suspend for 1 minute
- Enable "do not disturb" again
- Wait for > 1 minute, no reminder
- Disable "do not disturb" mode
- The reminder shows immediately

Reviewers: djarvie, broulik, davidedmundson

Reviewed By: broulik

Subscribers: winterz, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D25399