need calendar events plugin system

Please keep in mind that the current Gnome Clock applet is very slow to open as the user enables more calendars.
Performance isn't so much an issue as responsiveness, especially when the applet shows a whole lot more than just the calendar. This should be designed with that in mind, considering the agenda as a separate component from the date / time information, for example.

I would personally suggest this be considered a core requirement. Without indication of events it is half useless. I don't think responsiveness is a problem since events can be fetched in the background before the user opens the menu. Rather than pipes, I would suggest a basic DBus interface, which would not have the overhead of repeatedly spawning several processes and which would be more flexible in the long term.

@karl: would that make sense for testing the EDS integration? Otherwise, either a N+1 item, or a "bitesize" bug up for community contribution.

OK, this is a separate bug to; https://bugs.launchpad.net/indicator-datetime/+bug/542218

Essentially asking if we can have event plugins. Not wanting to sound dramatic at all but do we *REALLY* want to go out of our way to build a plugin interface when one already exists in evolution.

I think it's better to wait for evolution to improve performance wise than write another plugin system.

Calendar performance is certainly an issue, so we've excluded all but the "local source" for the present implementation, there are issues with the current branch which I think at the moment are somewhere deep inside of gdbus/dbusmenu but are probably not unfixable.

Testers/Debuggers welcome, a second set of eyes to review the changes wouldn't go amiss