idle-monitor: Change the API to be function-based

As the current API design has been unknowingly relying on an Xorg server
bug that may be fixed in the future, we need to modify the API so that
became-active is explicitly asked for by the user so that we won't get
flooded with a large number of events for almost every input event.

https://bugzilla.gnome.org/show_bug.cgi?id=688227