Bugzilla plugin is now converted to backend sync service

as the summary, Bugzilla is now a backend sync service. It can be used simply as it was a plugin before. When enable this service, any added task will be handled and attempt to sync bug information from specific related Bugzilla service.

This version works well, but not perfect. I'm being continue to add more features, such as the configuration ability.
Reason is simple: bad separation of concerns (UI and the logic for syncing with bugzilla are in the same layer) and back then missing backend infrastructure. Instead of constructing an object representing of the task and then pushing it through GTG core model, it updated it "on the fly". Because the sync thread is different from UI thread, you get into GTK issues and race conditions.

GObject.idle_add is a poor man's tool for synchronization: When UI thread has an idle time, please call this function with the following arguments..

If you use the existing sync infrastructure (or the new one), you avoid those GObject.idle_add