Use more g_signal_connect_object()

Most of these are probably paranoid, not bugs. I haven't done an exhaustive review to see what really needs it: that would be time-consuming and require significant familiarity with the Calendar codebase. This was just a quick check. Anyway, I vote for paranoid when it comes to using g_signal_connect() regardless. It's dangerous, and there's no reason to ever leave a signal connected after the user data object has been destroyed.

Fixes #423 (closed)