Draft of the new CalendarDecoration interface and first plugin ported (Picture of the Day). The interface still has to be extended to the various views, and implemented in the various views and positions (see my blog at <a href="http://blog.loic.corbasson.fr/">http://blog.loic.corbasson.fr/</a>; for an overview).

Picture of the Day is also still a work-in-progress: no functionality improvements there in this commit. Older plugins can still use the OldCalendarDecoration interface; places in the code relative to this interface have been marked to ease the removal if decided.

There is still a lack of documentation, which will be completed in a future commit. Feel free to comment, as this is my first interface design tentative, and finding a way to make it as flexible as wished in the initial specifications was not completely evident for me.