Add bindings for timed alarms, contributes to TJC#37226
This is a long-time request, see https://together.jolla.com/question/37226/calendar-let-user-select-reminder-time-for-events/
It allows to freely choose a reminder date and time. This is mostly convenient for full day events, but it can be used in many various ways to setup reminders non available in the combobox.
The support for this already exists in KCalCore and mKCal, thus also in the various sync plugin. It is just a question of bindings here.
I've added also a commit cleaning up a bit the spec file.