Gio.ListModel: implement most of the mutable sequence protocol. See #115
Adds all the dunder methods for MutableSequence to Gio.ListModel and Gio.ListStore.
__delitem__ supports atomic deletion of slices through splice() when
__setitem__ has to fall back to remove/insert since adding items with
splice doesn't work right, see https://bugzilla.gnome.org/show_bug.cgi?id=795307