tooltip: Get the pointer position relative to the toplevel widget

tooltip: Get the pointer position relative to the toplevel widget

Since we position the tooltip window relative to the toplevel widget and
not actually relative to the effective_toplevel, we shouldn't get the
pointer position relative to the effective_toplevel.

We previously used the pointer position (relative to the
effective_toplevel) and the anchor rect (relative to the toplevel
widget) together to calculate x_distance. This leads to wrong values in
cases where get_surface (new_tooltip_widget) != get_surface (toplevel)

Fixes #1427 in master