gcancellable: don't use g_cancellable_connect() in GCancellableSource

g_cancellable_connect() is documented as calling its callback only
once, but GCancellableSource should trigger every time the cancellable
is cancelled.

https://bugzilla.gnome.org/show_bug.cgi?id=710691
https://bugzilla.gnome.org/show_bug.cgi?id=711286