Remove pointless inout arguments

Remove pointless inout arguments

GtkTreeView.get_tooltip_context() takes an inout X and Y coordinates,
but the "out" side is a side effect: the conversion from widget-relative
to bin window-relative coordinates is not documented, and can be done
using public API, if needed.

GtkIconView.get_tooltip_context() follows the same pattern, and takes
two inout arguments for the coordinates, but it does not change them any
more, after GtkIconView's bin window was dropped in commit 8dc5e13e.

There's really no point in having these `inout` arguments, and while
GtkTreeView and GtkIconView are certainly de-emphasised in GTK4, and we
nudge developers to move to the new list views, we should take advantage
of the API break to remove warts.