[PATCHSET] Add reminders

Submitted by Olivier Brunel

Assigned to Florian Rivoal

Link to original bug (#14748)

Description

This is a patchset that adds reminders, i.e. alarms of sort that can be triggered on a given date & time. It also fixes issue where the date/time wasn't updated on resume, albeit only on Linux.

With it, it is possible to double-click on the calendar, to open a window where one can manage so-called reminders. A reminder is simply a date and time, a title, and optionally a (multi-line) description, a command line and/or a recurrence. On the specified date & time, the reminder is said to expire, and will be shown in a window where one can either ask to be reminded later (in a specified interval), or delete/apply recurrence. On start, all expired reminders are shown.

Reminders are indeed not kept once expired, nor do they appear multiple times when recurring. Instead, once expired they are simply either removed, or recurrence is applied, i.e. they're reset to a newly calculated date/time (based on the original one, i.e. not accounting for any "remind later" delay).

Recurrence is simply a given amount of time being added, meaning one cannot do more complicated things such as "first Monday of the month". However, a command-line can be specified for each reminder, which can take variables to get the reminders date/time, title, etc and it is possible to simply create new reminders by writing their definitions to stdout (which is piped to the plugin).

It is also possible to easily import/export reminders.

Note that this can all be disabled using --disable-reminders on configure for those who don't want the bloat :)