Make calendar used by ReplyMonitor configurable

[sphh]

Currently the ReplyMonitor creates a default calendar for its own purposes. This is a local calendar.

If you are using Thunderbird with ReplyMonitor on different computers, it is advantageous, to have this calendar synced between the different computers. A networked calendar would do the job.

Deleting the old calendar and creating a new networked calendar with the name "Reply Monitor" is not enough, because the calendar is referenced internally by ReplyMonitor by its ID and not its name.

Changing the calendar involves at the moment the following steps:

1. Creating the new networked calendar.
2. Opening the config editor.
3. Finding the ID of the newly created calendar.
4. Copying this ID into the calendar.replymanager.calendarID key.

My feature request would be a way to change the calendar used by ReplyMonitor in the settings window for ReplyMonitor.

Thanks.