if calendar source can be found, show calendar regardless of whether or not evolution's installed

[Charles Kerr]

This ticket comes from the idea behind Renato's MP at https://code.launchpad.net/~renatofilho/indicator-datetime/calendars -- "enable support for calendars events without evolution installation."

I've rejected that MP because we sat on it too long and the underlying codebase has changed too much, and am opening this ticket so that the idea doesn't get lost.