Allocate GObjectNotifyQueue with g_slice instead of abusing g_list

Allocate GObjectNotifyQueue with g_slice instead of abusing g_list

This is both cleaner and faster (it avoids function calls and
zeroing the memory twice).

Object construction performance improvement:
         Non-Threaded   Threaded
Simple:           11%       1.3%
Complex:           8%         6%

Other tests stable.

https://bugzilla.gnome.org/show_bug.cgi?id=557100