We named the argument `position` in the code and doc arguments, but the
rest of the documentation referred to `index` instead. That was maybe
meant to hint at the child property named :index, but we can simply be
fully clear here. We can call the argument `index_`, replacing the local
variable with that name, thus avoiding any possible confusion with the
unrelated ::get-child-position, and refer users to :index for completion
`index_` is used instead of plain `index` in case anyone is #including
<strings.h> and getting the old index() function superseded by strchr();
see https://gitlab.gnome.org/GNOME/gtk/merge_requests/932#note_531149
This patch changes all uses of GDK_DEPRECATED(_FOR) in gtk headers
by the versioned variants, GDK_DEPRECATED_IN_3_x(_FOR). At the same
time, we add GDK_AVAILABLE_IN_3_x annotations for all API additions
in 3.2 and 3.4.
